We investigated effects of frequent blood collections under the compulsory restraints on serum testosterone, LH and glucocorticoids in adult male Japanese monkeys. Blood sampls were withdrawn from three animals at 30-min intervals and two animals at 4-hr intervals during 24 hr. Serum testosterone and LH were measured by a radioimmunoassay, and glucocorticoids was determined by a competitive protein binding assay. Sampling at 4-hr intervals during 24 hr revealed diurnal changes in serum testosterone and glucocorticoids. Levels of testosterone were high throughout the night and low at the day time, conversely serum glucocorticoids were high levels in the morning and low in the evening. On the other hand, 30-min intervals sampling, serum testosterone levels decreased and glucocorticoids levels increased, respectively, immediately after the start of blood sampling. And then, low in testosterone and high in glucocorticoids levels were continually maintained. But serum LH levels had the pulsatile pattern and did not change markedly during 24 hr. After ACTH administration in five animals, serum glucocorticoids levels increased markedly and also testosterone levles increased slightly, but LH levels did not change. These data indicated that the every 30-min restraint stress caused the increment of glucocorticoids levels and the suppression of testosterone levels, but did not affect the serum LH levels. The increased glucocorticoids might inhibit the testicular steroidogenesis, without suppressing the LH secretion from the pituitary.