Post-partum plasma progesterone concentrations and luteal progesterone content were measured in lactating and non-lactating ewes during a normal breeding season in autumn and an abnormal breeding season in spring. The results indicate that silent ovulations during the autumn breeding season were accompanied by decreased plasma progesterone concentrations and the absence of oestrus. Furthermore, oestrus activity was probably inhibited by lactation, but the latter had no influence on ovulation. During spring, ovulation and oestrus were delayed. It was concluded that post-partum ewes can be tupped earlier during an autumn breeding season, than during spring.