Abstract. The effect of castration on pituitary androgen and estrogen nuclear receptors was examined in eleven 3-year-old 'Préalpes du Sud' rams which were either intact (N = 5) or surgically castrated (N = 6) 11 months before. To avoid a receptor loss, pituitaries were frozen less than 1 min after slaughter. Specific androgen and estrogen bindings were measured upon extracts from purified nuclei. Neither the concentration of estrogen receptors (30.5 ± 3.0 vs 28.1 ±3.0 fmol/mg protein) nor the affinity constant (ka = 1.21 × 109 mol−1 in both groups) differed between intact and castrated rams. Conversely, androgen receptor concentrations differed markedly between groups and were found significantly higher in intact rams (11.3 ± 1.1 fmol/mg protein) than in castrated rams where nuclear receptors were undetectable in 5 out of 6 animals. This result keeps open the possibility that the decrease of LH sensitivity to testosterone negative feedback observed after long-term castration, could be related to the quasi absence of nuclear receptors, at least at the pituitary level.