The influence of thyroid deficiency and the administration of thyroxine on pituitary-testicular function were studied in male albino rats from weaning age (22 days old) up to 82 days of age. The results showed that the hyperthyroid state induced by a daily injection of 2.5 or 5 microng L-thyroxine resulted in acceleration of growth, a comparative increase in size and number of spermatogenic and interstitial cells, an increase in the STH cells, particularly at the earlier age (42 days old), and in a decrease in the number and size of TSH cells. Gonadotrophic FSH and LH and prolactin cells exhibited an increase in their granular content. The hypothyroid state induced by thyroidectomy or thiourea feeding, at the levels of 0.1 and 0.2% resulted in the depression of growth rate, destructive changes of the spermatogenic and interstitial cells and also in the lumen of the seminiferous tubules. A decrease in the STH, gonadotrophic FSH and LH and prolactin cells and hypertrophy of TSH cells accompanied by degranulation were also observed.