Previous experiments with inbred mice showed that chronic ethanol treatment delays male pubertal development. An initial event in sexual maturation in the rat is a transient increase in 5 alpha-reductase. The present study was conducted to determine whether similar ethanol effects occur in outbred mice (Swiss-Webster), to determine the ontological profile of testicular 5 alpha-reductase in the mouse, and to evaluate the effect of ethanol treatment on this enzyme. After 29 days of treatment with a liquid diet (beginning at age 20 days), reductions in the ethanol-treated mice as compared with the controls were noted in testicular weight (55.0 +/- 2.0 vs. 63.0 +/- 2.4 mg; P less than 0.01), epididymal sperm content (6.8 X 10(5) vs. 14.4 X 10(5); P less than 0.05), and sperm motility (45% vs. 57%; P less than 0.05). After 43 days, differences no longer existed. In chow-fed mice, a substantial rise in 5 alpha-reductase (1 unit = 1 pmole DHT formed/45 min/mg testis) began at age 24 days. Activity peaked at approximately 65 units at 25 to 30 days and gradually declined to 6.4 +/- 0.8 units at 63 days. After 29 days treatment, 5 alpha-reductase of the pair-fed control group was 26.8 +/- 4.9 units, which decreased to a baseline value of 7.0 +/- 2.1 units after 43 days treatment. In contrast, 5 alpha-reductase of the ethanol-treated group remained at baseline levels after 29 days (7.7 +/- 2.3 units) and 43 days of treatment (7.6 +/- 2.3 units).(ABSTRACT TRUNCATED AT 250 WORDS)