Diurnal changes of heart rate obtained by continuous 24 h (Holter) ECG in 5 beagle dogs during anoestrus, pregnancy and lactation. For simplicity only the means are shown (for means and SEM, see Table 2). The heart rate was significantly (p < 0.001) increased already at 3 weeks of pregnancy compared to anoestral values and was further increased at 5 and 7 weeks of pregnancy. At 4 weeks of lactation, the heart rate had decreased to a level similar to that at 3 weeks of pregnancy. Bpm = beats per minute.