Recent reports have demonstrated that plasma testosterone shows a significant diurnal variation, with an early morning peak (Resko & Eik-Nes, 1966; Faiman & Winter, 1971; Okamoto, Setaishi, Nakagawa, Horiuchi, Moriya & Itoh, 1971). Testosterone has also been reported to fluctuate during sleep, possibly associated with rapid eye movement episodes (Evans, MacLean, Ismail & Love, 1971). Thus testosterone, like cortisol (Hellman, Nakada, Curti, Weitzman, Kream, Roffwarg, Ellman, Fukushima & Gallagher, 1970; Krieger, Allen, Rizzo & Krieger, 1971) may be secreted episodically. In a recent study of ten young men, aged 19–23 yr, who were closely observed while participating in their usual daily routines, plasma samples were drawn approximately every 90 min during the day and night from indwelling catheters. Cortisol was analysed in all samples by the method of Murphy (1968), and testosterone was analysed in all men every 6 h by a modification of the method of Mayes & Nugent
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Rose et al. (1972) studied this question.