The endocrine profile of FK 33–824, 0.5 mg i.m., was determined in comparison with placebo or pretreatment with naloxone, 4 mg i.v., in 14 healthy men. Prolactin (PRL), growth hormone (GH), ACTH, cortisol, gonadotropins(LH, FSH), thyrotropin(TSH), thyroxine (T<sub>4</sub>), triiodothyronine (T<sub>3</sub>), insulin and glucagon were measured and free water clearance was calculated from urine volumes and osmolar clearances. FK 33–824 increased PRL and GH (p < 0.001). ACTH was below assay sensitivity but cortisol was significantly lowered (p < 0.001). Free water clearance was enhanced (p < 0.01) and the remaining parameters were unchanged. Naloxone alone had no hormonal effect but abolished the increase in PRL and GH following injection of FK 33–824 without modifying the decrease in plasma cortisol or the increase in free water clearance following the same treatment. The results indicate that the effect of FK 33–824 on PRL and GH release is mediated by opiate receptors. Other mechanisms or naloxone nonsensitive receptors may be implicated in the effects recorded on cortisol and FWC.