SUMMARY The structure of interstitial cells and the cells of the lobule wall was studied in the lamprey testis throughout the period of sexual maturation. In the course of their secretory cycle the interstitial cells showed the usual characteristics of vertebrate steroidogenic tissues; a well-developed agranular reticulum, mitochondria with a dense matrix and tubular cristae, and lipid inclusions. Similar characteristics were also seen in the cells of the lobule wall in the period immediately preceding spermiation. In the interstitial cells, hydroxysteroid dehydrogenase (HSD) activity reached a maximum in late January and February, in advance of the development of the secondary sex characteristics, and was correlated with the abundance of interstitial lipid. After hypophysectomy, no changes were detected in the numbers or volume of interstitial tissue, and HSD activity was apparently unaffected. Characteristic multivesiculate bodies were not observed in the cells of the lobule wall in hypophysectomized animals.