Fig. (1) Schematical relation between hypothalamic–pituitary–adrenal axis and hypothalamic–pituitary–gonadal axis in men. Note the intrincate web of these axes. ∆t indicates the putative link between serum cortisol level and its direct inhibition of testosterone production in gonads, a link responsible for the testosterone biphasic behavior described in the text. References that report non-classical couplings are given on the respective arrows. ANS: autonomic nervous system. ~ Master: master oscillator (circadian rhythm). ~ local: local oscillator. Dashed line: inhibition. Continuous line: stimulus.