Oestrous activity was monitored at two locations (Invermay in Otago and Woodlands in Southland) in ewe flocks of similar age structure, synchronised at the beginning of the breeding season each year for 5 years . After an 11 -day CIDR treatment, interval to oestrus, duration of oestrus, and ovulation rate were measured. Age of ewe affected the mean interval to oestrus (32.4-41.0 h), with 2-tooths commencing up to 10 h later than older ewes (P < 0.05). There was a significant location effect in 1992 when oestrus commenced 8.6 h later at Woodlands than at Invermay (41.0 versus 32.4 h; P < 0.05). Duration of oestrus differed significantly between some years (P < 0.05) at Woodlands but not at Invermay. There was also a difference between locations whereby Woodlands animals had a longer oestrus (23.4-34.2 h), by 7.4-17.5 h (P < 0.001), than those at Invermay (15.0-18.1 h). This difference did not occur in 1995, when both flocks were located at Woodlands. Mean ovulation rate (1.9-2.2) was not significantly a Corresponding author A96056 different either within or between locations. These data suggest that even in the same flock, age of ewe, year, and location may influence oestrous activity in ewes treated to manipulate the oestrous cycle at the beginning of the breeding season.