The effects of sex steroids on luteinizing hormone (LH) secretion in rabbits of both sexes castrated for at least 2 weeks previously were investigated. An intramuscular (i.m.) injection of testosterone propionate (TP) or estradiol benzoate (EB) was given about midway through blood sampling which ranged from 6 to 12 h. LH and steroids were measured by established radioimmunoassay (RIA) procedures. Variable responses were obtained with animals. Castration itself in most instances led to increases in circulating levels of LH which ranged from normal levels to greater than 200 ng/ml with some evidence for episodic variation. EB was more effective than TP in reducing LH levels and inhibiting episodic release.