Nandrolone decanoate (Deca-Durabolin) was injected intramuscularly into healthy volunteers. One group of females received one injection of 100 mg and three groups of males received one injection of 200 mg, two repeat injections of 100 mg or four repeat injections of 50 mg respectively. The serum levels of nandrolone (19-nortestosterone) were determined by radioimmunoassay and used to estimate pharmacokinetic parameters. The following pharmacokinetic parameters were found: a mean half-life of 6 days for the release of the ester from the muscular injection depot into the general circulation; a mean half-life of h for the combined processes of hydrolysis of nandrolone decanoate and of distribution and elimination of nandrolone; a mean nandrolone serum clearance of 1 X h-1 X kg-1. The half-life of hydrolysis of nandrolone decanoate in serum was of the order of one hour or less. The data are consistent with linear kinetics.