Plasma E2 concentrations and its variability between animals recorded before treatment in the present study (range from 13.7 to 44.7 pmol l-1) are consistent with previous reports in llamas [6, 37, 38]. This observation indicates animals were at different stages of follicular development at the time of treatment, as it has been established a close relationship between follicular activity and E2 secretion in llamas [5, 6, 38, 39]. In addition, the plasma E2 concentration established as the physiological upper limit in the present study was somehow in agreement to that previously registered during the follicular phase (17 to 47 pmol l-1) in llamas . The fact that plasma P4 concentrations remained bellow 3 nmol l-1 in all cases indicates that no ovulations occurred during the study .
To our knowledge, this is the first report showing the plasma E2 profile following a single i.m. injection of ECP and its comparison to the profile obtained after injection of EB in llamas. Treatment with both oestradiol derivatives induced plasma E2 concentrations exceeding those considered physiological for the species. Moreover, the plasma E2profiles recorded in the present study clearly depended on the ester administered and were consistent with that registered in cows by Vynckier et al. . Plasma E2 concentrations increased rapidly and peaked earlier in EB group with values three-times higher than in ECP group. Similar results were previously observed after EB injection in bovines [21, 22].
Peak plasma E2 concentrations achieved after EB injection in the present study were higher than those recorded before in camelids [24, 31] and were attained earlier than observed by others  in alpacas. Although there is no clear explanation for the divergences registered between studies, it could be speculated that differences in blood sampling schedule, sensitivity of hormone assays and formulation of hormonal products, may be responsible for the discrepancy.
Once peak plasma E2 concentrations were reached, the declination rate also differed between esters. The biphasic decline of plasma E2concentrations until physiological values on day 5 in EB group is in agreement with previous reports in alpacas and cows [24, 35]. As stated by Vynckier et al.  these biphasic decline suggest an initial phase of distribution followed by an elimination phase. In contrast, in ECP group, the decreasing phase of the curve was shorter and plasma E2 concentrations fluctuated close to physiological values from day 9 onwards. Although the pattern of decline observed for ECP in our study is consistent with that recorded in cows by Vynckier et al. , they showed that concentrations returned to physiological values slightly earlier (day 7) in this species. The slower return to physiological values in ECP compared to EB group indicates a longer half-life for ECP as has been registered previously in cattle [17, 19, 35].
From the analysis of plasma E2 concentrations, it is apparent that ECP was absorbed at a lower rate than EB, but persisted elevated in plasma for a longer period (8 and 4 days, respectively). Besides the different rates of absorption, the same amount of drug was released from the injection site for both esters, since by the end of the study the AUCs values were similar between groups. These findings are consistent with the fact that the esterification process prolongs the half-life of E2. Apparently, the ester injected influences the mode that E2 is released from the injection site. Thus, there is an inverse relationship between the extension of the ester chain and its water solubility and consequently, on its absorption rate. Once in circulation, the esters are hydrolyzed to E2, the active hormone. Consequently, the duration of action appears to depend on the rate of absorption more than the metabolism .
It has been suggested in llamas that hormonal treatments with progestogens are effective in completely prevent follicular development for a period of up to 7 days with the lowest follicular activity registered between days 5 and 7 after beginning of treatment [32–38]. Thereafter, most attempts to control follicular activity includes short protocols (5 to 7 days) and a combination of a progestogen and oestrogens. According to the results hereby presented, E2 plasma concentrations would remain at pharmacological concentrations beyond the day of progestogen withdrawal after ECP treatment. Conversely, plasma E2 concentrations would have return to physiological values before the end of the synchronization protocol after EB injection. Whether this might influence the outcome of the synchronization treatment or not remains to be elucidated.