Studies were made to determine if a substance other than melatonin was present in bovine pineal gland extracts which possessed properties inhibitory to some gonadotrophin-dependent reproductive parameters in rats. Bovine pineal extracts were partially purified by organic solvent extraction or by ultrafiltration and gel-filtration to exclude melatonin. When administered to rats, the extracts inhibited compensatory ovarian hypertrophy, delayed vaginal opening time and reduced the incidence of constant oestrus. Either of the melatonin-free extracts reduced significantly the concentration of serum luteinizing hormone (LH) in long-term castrate male rats. The data suggest that some substance other than melatonin present in bovine pineal glands has a modifying effect on the reproductive parameters observed probably due to its inhibitory effect on LH.