Abstract
The cyclic secretion of oestrogens and progestogens during the rat oestrous cycle has been well established (Brown-Grant, Exley & Naftolin, 1970; Barraclough, Collu, Massa & Martini, 1971; Shaikh, 1971); however, virtually nothing is known concerning variations in plasma androgen concentrations. In order to determine whether cyclic fluctuations in androgen levels occur, peripheral plasma levels of testosterone and androstenedione, in conjunction with oestradiol concentrations, were measured at various times during the 4-day rat oestrous cycle. Adult (60–90 days) virgin female Sprague—Dawley rats (Madison, Wisconsin) were maintained under controlled atmospheric and lighting conditions (lights on from 05.00 to 19.00 h). Vaginal smears were taken daily and only rats demonstrating at least three consecutive 4-day cycles were used. The duration of the critical period for the release of the ovulatory surge of gonadotrophins was established by injection of sodium pentobarbitone (30 mg/kg body weight, i.p.) on the afternoon of pro-oestrus. An injection given