Serial prostatic specific antigen values (PSA) were determined on 42 patients receiving definitive radiation therapy for localized adenocarcinoma of the prostate. The PSA declined exponentially in 25 patients. None of these patients experienced metastases. The PSA initially declined then increased exponentially in 17 patients. The rate of decline was similar to the rate of rise in all 17 patients. Five of these patients had distant metastases (P less than 0.02) within 2 years of treatment. The PSA values after radiation therapy were employed to formulate a model of tumor kinetics. This model predicts the mean duration of G0. This parameter is correlated with the development of distant metastases within 2 years of treatment. For those patients at low risk for relapse, the mean G0 is calculated to be 22.5, and 13.6 weeks for those who have relapsed or are at high risk for relapse.