Groups of 6 adult male rats were killed at 4-h intervals over 24 h periods. The serum samples were assayed for LH, FSH, prolactin (Prl) and testosterone (T). LH, FSH and lactogen receptors were measured in the testis homogenates. In the 2 experiments performed, statistically significant (P less than 0.01-0.05) positive correlation was observed between serum LH and T levels, as well as between serum LH and testis LH receptor concentrations. Significant (P less than 0.05) diurnal variation was seen in peripheral serum T (high levels at 08.00-12.00 h, low levels at 16.00-20.00 h) and LH (high at 24.00-08.00 h, low at 12.00-20.00 h), whereas FSH and Prl levels did not display significant variations over the period of 24 h. The testicular content of LH receptors showed significant (P less than 0.05) diurnal periodicity in the 2 experiments performed. Maximal binding measured in each experiment was between 04.00 and 08.00 h. The low binding was in one experiment between 12.00 and 20.00 h, and in the other between 20.00 and 24.00 h. No consistent variation was measured in testicular FSH or lactogen receptors. These results suggest that, in addition to differential stimulation of steroidogenesis, physiological variations in gonadotrophin levels can induce autoregulatory changes in the testicular gonadotrophin receptors.