Gunn & Gould (1958) introduced evidence of inherent testicular rhythms in the laboratory rat. They reported that the uptake of radioactively labelled zinc by the dorsolateral prostate (DLP) during the span of 1 year shows two distinct maxima, in February and in June. Further support of this concept came from studies of androgen biosynthesis by the rat testis in vitro (Ellis, 1970). Recent investigations in this laboratory reveal additional, more direct, information concerning rhythmic endocrine performance and also the possibility of a rhythm in gametogenic function of the rat testis. Male Sprague—Dawley rats, 4 weeks of age, were housed individually in suspended wire-mesh cages at room temperature (21 ± 1 °C) and relative humidity (45–65%). The daily lighting cycle was 12 h light: 12 h darkness (lights on 06·00–18·00 h) and the animals received food and water ad libitum. Functions of the testes were subsequently examined with groups of six