Immunization of ewes against a pure recombinant preparation of the alpha subunit of bovine inhibin (alpha-bI) resulted in a three- to fourfold increase in ovulation rate, associated with antibodies in plasma recognizing pure native 31 kDa inhibin. The aim of this study was to examine the effects of this immunization on basal and GnRH-stimulated plasma concentrations of FSH and LH in ewes during the anoestrous and breeding seasons. The groups were untreated control ewes (n = 5), control ewes treated with keyhole limpet haemocyanin (KLH alone, n = 4), ewes treated with alpha-bI alone (n = 4) and alpha-bI-KLH conjugate-treated ewes (n = 3). There were no effects of immunization on basal FSH or LH in anoestrous ewes, despite the presence of antibodies recognizing 31 kDa inhibin. In the breeding season, immunization against alpha-bI resulted in increased basal (follicular phase, P less than 0.1; luteal phase P less than 0.05) and GnRH-stimulated (follicular phase only, P less than 0.001) release of FSH, but not LH. The data are compatible with the hypotheses that the increase in ovulation rate in immunized ewes is due to an increase in circulating FSH concentrations and that inhibin may only have a major peripheral influence on FSH in sheep during the breeding season.