This research was conducted at the La Raya experiment station, Cusco, Peru (4200 m elevation, 15°S latitude, and 70°W longitude) using 60 Corriedale, and 60 Criollo ewes during three consecutive years (April 1981 - March 1984). Age, and weight at puberty, age at physical maturity, and the effects of breed, age (physically immature and mature), year, and month were measured on body weight, incidence of estrus, and incidence and rate of ovulation. There was a significant difference (P.05). Even though Criollo e wes weighed less than Corriedale, they attained puberty earlier, and showed less seasonality in estrus and ovulated throughout the year than the Corriedale.