Measurements are reported on the rates of oxidation of 14C-1-palmitate, 14C-1-pyruvate, and 14C-2-pyruvate by cell suspensions obtained from testes of normal rats of varying ages and from hypophysectomized regressed rats. Highest conversion rates of labeled pyruvate to CO2 were observed in testes from 24-day-old rats, in which all germinal cells except spermatids and spermatozoa were present. Cell suspensions from testes of adult rats, containing predominantly spermatids, had relatively low rates of palmitate and pyruvate oxidation. These rates were increased in cell suspensions from testes of regressed hypophysectomized rats towards those observed in testicular cell preparations from immature rats. The predominant cell types in testes from hypophysectomized, regressed rats are spermatogonia and early spermatocytes, although early stage spermatids are also present in lesser numbers. The ketogenic enzyme capacity was greatest in particulate preparations obtained from testes of normal 14-day-old rats, in which the predominant germinal cells present are spermatogonia. The activity of succinyl-CoA: 3-oxoacid CoA-transferase was also highest in these preparations. Cell suspensions from testes of 14-day-old rats incorporated significant amounts of labeled palmitate and pyruvate into acetoacetate, whereas cell suspensions from testes of other groups of animals examined did not. The data are discussed in relation to factors controlling rates of fatty acid oxidation in various germinal epithelial cells. It is concluded that spermatocytes have highest rates of pyruvate oxidation, but that both spermatogonia and spermatocytes have relatively high rates of palmitate oxidation. Since spermatogonia also were shown to contain the relatively highest ketogenic enzymic capacity, and since these cells had previously been observed to have lowest levels of carnitine acetyltransferase (CAT), it may be deduced that high CAT activity is not required for fatty acid oxidation or ketogenesis by testicular cells.