The present study was undertaken to investigate effects of oxytocin administered subcutaneously at early (days 4, 5 and 6) and late (days 14, 15 and 16) luteal phase of the oestrous cycle, on luteal regression.

A total of 20 cows (18 Brown Swiss and 2 Holstein), aged 3-8 between years were used in this study. Their oestrous cycles were synchronised by intramuscular administration of 25 mg Dinoprost trometamin twice at 11 day intervals. The animals were then randomly divided into two groups, A and B. In the group A, on the days 4, 5 and 6 of the first oestrous cycle, 20 ml physiological saline was subcutaneously injected to the animals at two divided doses with 12 hour intervals. Oxytocin (100 IU) at two divided doses (12 hour intervals) on the same days of the second oestrous cycle was administered subcutaneously. The same treatments were applied to the animals in the group B on the days 14, 15 and 16 of the oestrous cycle. Blood samples were collected from all animals at two daily intervals during the two oestrous cycles. Serum progesterone levels were measured by double RIA.

One animal in the group A had a shortened oestrous cycle following administration of oxytocin at the early stage. Another cow in this group did not show symptoms of heat on the day 32. The lenght of the oestrous cycles in the remaining eight animals as not affected by oxytocin injections. In the group B which received oxytocin at late luteal phase, one animal had a prolonged oestrous cycle. Another cow in this group did not show symptoms of heat even on the day 32. Lenght of the oestrous cycles in the remaining eight animals was not affected by oxytocin. .

In the group A, on the days 4, 6, 10 and 12 of the second oestrous cycle, serum progesterone levels were significantly lower than those of the first cycle. In the group B, on the days 8 and 18 of the second oestrous cycle, serum progesterone concentrations were higher than the same respective days during the first oestrous cycle. .

In summary, it was concluded from the present study that administration of oxytocin at doses and durations indicated above did not influence significantly the length of oestrous cycle and serum progesterone levels in cows.