Suppression of luteinizing hormone (LH) by sc implanted oestradiol-17 beta (E2) pellets was examined in 4 ovariectomized female rhesus monkeys during the breeding season, the non-breeding season and during the transition between the breeding and non-breeding season. Immunoreactive LH was suppressed to 58, 78 and 75% of untreated levels for the respective seasonal conditions. Bioactive LH was suppressed to 29, 49 and 33% of baseline. Bioactive LH (determined by testosterone release from rat interstitial cells) was significantly correlated (r = 0.84) with immunoactive LH from the same samples. It is concluded that E2 treatment of ovariectomized female rhesus monkeys results in suppressed levels of LH, regardless of the time of year.