ABSTRACT It has been shown that the duration of action as well as the ratio of anabolic and androgenic activities of nandrolone esters depend on the rate at which they are absorbed from an intramuscular depot. Following release from the depot the nandrolone esters are rapidly hydrolyzed in the blood and the resulting nandrolone was found to be the ultimate active compound affecting m. levator ani and seminal vesicles. However, the rate and concentration at which nandrolone reaches these receptors is fully determined by the rate of absorption of the ester. This dependence explains why nandrolone released from its esters in vivo has pharmacological effects quite different from those occurring after administration of nandrolone in a single dose or in equally divided doses. The findings further demonstrate that the rate at which a drug enters the body and particularly the variations of this rate in the course of time, not only determine the intensity and duration of action but also change its pharmacological pattern. The latter conclusion is not limited to the nandrolone esters but can be expected to hold true for other drugs as well.