Post-partum changes in the concentration of progesterone, oestradiol-17β and prolactin were measured by radioimmunoassay in the blood plasma and milk of 28 buffaloes. The mean concentration of progesterone in the plasma of normal animals was 0·24 ± 0·04 μ.g/1 on the day of calving and decreased linearly during the post-partum period, until the complete regression of the residual corpus luteum of pregnancy occurred, when the plasma concentration was recorded as 0μ05 to 0μ08 μg/l. The concentration at a silent oestrus was 0μ17 ± 0·02 μ.g/1. The milk progesterone profile was parallel to that of plasma but the level was 2 to 4 times higher.
The mean concentration of oestradiol-17β in plasma was 41·0 ± 2·34 ng/1 on the day of calving; it declined significantly by day 1 post partum (P < 0·001) and fluctuated between 8 and 16 ng/1 until oestrus, when the concentration was approximately 25 ng/1. The oestradiol-17β concentration in milk was significantly higher than that of plasma on the day of calving. During the first 20 days post partum, a similar concentration of oestradiol-17β was maintained in both fluids but thereafter the concentration in milk was 2 to 3 times that of plasma.
The mean concentration of prolactin in plasma was 212·6± 19·08 μg/1 on the day of calving and then declined and fluctuated between 70 and 270 μg/1 during the rest of the period. A minor elevation occurred around oestrus. The prolactin concentration for the first 2 days post partum in milk was significantly higher than that in plasma but during the rest of the period it was similar to that in plasma.
The progesterone concentrations were slightly lower in anoestrous animals than in those cycling. The concentration of prolactin was significantly higher in multiparous animals (P < 0·001), whilst the progesterone and oestradiol values were similar in primiparous and multiparous animals.