Within 12 hr after castration, there is a dramatic drop in the serum testosterone (T) levels to approximately 1.3% of the intact value (2.5 +/- 0.8 ng/ml). By 1 day following castration, the serum T levels are approximately 3.3% of the intact control level. In contrast, serum 5 alpha-dihydrotestosterone (DHT) levels decrease to only 50% of the intact value within 12 hr postcastration and remain at a value greater than 50% of the intact control level even following long-term castration for up to 20 weeks. Following castration, tissue T and DHT concentrations in rat ventral prostate (RVP) exhibited a similar sequence of changes. Within 12 hr after castration, there is a substantial decrease in T to 27% and DHT to 20% of their intact values; after a further transient decrease during the subsequent 7 days, these levels remain constant with RVP at approximately 40% for T and 20% for DHT of the intact control levels even following long-term castration. Thus castration induces only a partial withdrawal of the tissue androgens. The low but measurable androgen levels in RVP of castrated host are of adrenal origin, since following surgical adrenalectomy these remaining androgen levels become undetectable. Thus castration plus adrenalectomy produces a complete androgen withdrawal within the RVP. To determine the biological significance of the measurable androgen levels remaining following castration, the RVP cell number and the rate of prostatic DNA synthesis were compared in RVP following castration alone (ie, partial androgen withdrawal) or castration combined with surgical adrenalectomy (ie, complete androgen withdrawal). These results demonstrated that complete elimination of the remaining androgens in the RVP of long-term castrates, by means of surgical adrenalectomy, did not induce any further reduction in either of these prostatic growth parameters. Therefore, in the rat, DHT must be decreased to a critical threshold but does not have to be completely eliminated to decrease maximally androgen effect on the prostate.