Investigations on the mechanism of colostrum effect in sheep sex apparatus stimulation were carried out. The experiments included 57 sheep, 10 of them used as a check and the remaining 47 distributed in four treatment groups and injected with colostrum at various phases of the sex cycle. It was established that the effectiveness of injecting cow colostrum on sheep with the aim of inducing synchronous oestrus during the regular reproduction season depends on the sex cycle phase at which the injection takes place. The effectiveness is higher in case injection is performed during the second half of dioestrum. The induced synchronous oestrus of sheep following colostrum injection is equivalent to the normal.