Three adult male rhesus monkeys were castrated and administered 2 mCi [3H]dihydrotestosterone ([3H]DHT) intravenously. Brain and peripheral organs were removed after 60 min and were examined either by thaw-mount autoradiography or by subcellular fractionation and high performance liquid chromatography. Neurons that accumulated radioactivity in their nuclei were distributed widely in many regions of the brain including the preoptic area, hypothalamus, septal area-bed nucleus, amygdala, thalamus, and brain stem. Several brain areas which were labeled after [3H]DHT injection had not been labeled in earlier experiments after [3H]testosterone ([3H]T) injection. The major metabolite of [3H]DHT in extranuclear fractions from brain was [3H]androstanediol, but [3H]DHT alone was detected in cell nuclei. There was no evidence of any [3H]estradiol in cell nuclei, confirming that DHT can also be regarded as a non-aromatizable androgen in the primate brain. Since the nuclear concentrations of androgens were 2-3 times higher in the brain following [3H]DHT than they were in the earlier [3H]T experiments, the relative lack of effectiveness of DHT in restoring the sexual behavior of male castrates cannot be related to an inability of DHT to enter brain cell nuclei.