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Department of Physiology and Biophysics, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada, B3H 4H7

(Received 22 April 1977)

The concentration of testosterone in the serum of the adult rat varies significantly over a 24 h period (Kinson & Lui, 1973; Howland, 1975; Wilson, McMillan, Seal & Ahmed, 1976). However, studies on circadian variations in serum testosterone concentrations in immature male rats have yielded conflicting results. Grotjan & Johnson (1976) reported significant changes with time in 25–26-day-old Holtzman rats, whereas Döhler & Wuttke (1976) did not observe significant changes in 13–18 or 25–30-day-old Sprague–Dawley rats maintained on the same lighting schedule (14 h light: 10 h darkness).

Recently we reported that from 20–35 days of age, 5-androstane-3,17β-diol (androstanediol) is the predominant androgen in the circulation of male rats (Moger, 1977). This study was undertaken to determine temporal variations in the concentrations of androstanediol, androsterone and testosterone.

Forty-nine male Sprague