The steroid 5 alpha-reductase enzyme catalyzes the conversion of testosterone to the potent androgen 5 alpha-dihydrotestosterone (DHT). PNU 157706, a novel, potent and selective dual 5 alpha-reductase inhibitor, was reported to be effective in inhibiting the growth of established tumors in the Dunning R3327 rat prostatic carcinoma model. We have studied the efficacy of combined treatment with PNU 157706 and the antiandrogen flutamide in this prostatic tumor in rats. Rats with tumor diameters of about 1 cm were treated orally 6 days a week for 9 weeks with PNU 157706 (10 mg/kg per day) alone or in combination with flutamide (1 and 5 mg/kg per day). Animals were killed 24 h after the last treatment and ventral prostates were removed for testosterone and DHT determination. PNU 157706 reduced the growth of established tumors by 36%; flutamide showed a slight effect at 1 mg/kg per day (24% inhibition), while at the dose of 5 mg/kg per day it reduced tumor growth by 48%. The combination of PNU 157706 with the lower dose of flutamide caused an additive tumor growth inhibition (60%) and the combination with the higher dose of flutamide resulted in a better inhibition of tumor growth (68%) than did either treatment alone. Castration resulted in marked tumor growth inhibition (76%). Ventral prostate weight was more markedly reduced by PNU 157706 treatment than by flutamide; combined treatment was as effective as castration. Prostatic DHT content was markedly reduced by PNU 157706 (93%), whereas prostatic testosterone increased (137%). Concomitant treatment with flutamide partially antagonized the testosterone increase induced by PNU 157706 and did not modify the already considerable suppression of DHT. These data show that the inhibitory effects of PNU 157706 and flutamide on Dunning prostatic tumor growth are additive, thus supporting the rationale of this combination therapy in advanced prostate cancer, in order to achieve adequate androgen blockade with minimal side-effects.