The postpartum changes in plasma leptin levels of dairy cows ovulated within 35 days after calving were compared to of those with delayed onset of cyclicity in two experiments. The cows were sampled for leptin, insulin, IGF-1, thyroid hormones (T4, T3, rT3), Cortisol and also for some metabolites (glucose, NEFA, BHB and TCH) on day 1-3 after calving and again four times 7 days apart (Exp. 1), or in weeks 1, 2, 3, 5 and 10 postpartum (Exp. 2). In Exp. 1 also the standard low dose ACTH-induced Cortisol response and TRH-stimulated T4/T3 responses were determined on days 1-3 and 28-35. In cows of Exp. 1 the leptin level varied within a wide range with a mean of about 4 ng/ml HE in the first samples. Up to week 5 it remained unchanged in cows ovulated within day 35 but reduced in those with delayed onset of cyclicity, resulting in significant difference between the two group means.

In cows of Exp. 2 the week 1 leptin levels were lower and less variable than that of Exp. 1. In weeks 2 and 3 a slightly increasing tendency of leptin pattern was seen in cows resuming their ovarian cyclicity within 35 days whereas in those with delayed onset of cyclicity the leptin level remained unchanged. Up to week 5 this tendency resulted in exactly the same leptin values as seen in Exp. 1 with significant differences between the group means in both of the weeks 5 and 10. It was concluded that the plasma leptin concentration in postpartum dairy cows may interfere with resumption of cyclic ovarian function, but its influence may be only permissive (e.g. a minimal concentration above a supposed threshold seems to be the prerequisite of the onset of cyclicity)