The plasma LH level of ewes which did not exhibit oestrus during lactation was not significantly different from the inter-oestrus level of ewes showing regular oestrus. The LH level in the plasma of ewes kept isolated from rams was significantly lower than amongst similar, but anoestrous, ewes continuously associated with rams. In ewes which were cycling regularly the basal LH level remained relatively constant from day to day during inter-oestrus whereas in anoestrous ewes the daily samples showed considerable fluctuation.