Intravaginal pessaries, each containing 40 mg 6α-methyl-17 α-acetoxy-progesterone, were inserted into 80 New Zealand Romney ewes. Pessaries were removed after 14 days and some ewes subjected to various sequences of injections with pregnant mares’ serum gonadrotrophin (PMSG, up to 3 treatments). PMSG administration on the day of pessary withdrawal compared with “no PMSG” did not significantly affect the distribution of the onset of oestrus at the first oestrous period. However, the onset of the second oestrus after pessary withdrawal occurred after a significantly longer period if ewes were given PMSG immediately after progestagen withdrawal. The effect was magnified at the third oestrous period. Control ewes, given progestagen alone, maintained synchronisation of oestrus over the three oestrous periods. An interaction between PMSG and progestagen and a possible luteotrophic effect of PMSG are suggested as reasons for disruption of synchronisation.