Testes of sexually mature Sarotherodon mossambicus were incubated at 15, 22, 30, and 40 degrees with (a) tritiated testosterone and (b) salmon pituitary extract. Formation of 11-keto- and 11 beta-hydroxytestosterone from the tritiated precursor showed little change in yield between 15 and 30 degrees but yields of glucuronides rose dramatically between 22 and 30 degrees and a significant rise was observed for formation of 5 beta-androstane-3 alpha, 17 beta-diol between 15 and 40 degrees. Yields of 3 alpha, 17 beta-dihydroxy-5 beta-androstan-11-one followed a pattern similar to that of 11-ketotestosterone. With endogenous precursors under the stimulation of salmon pituitary extract, yields of testosterone, 11-ketotestosterone, and 11 beta-hydroxytestosterone were maximal at 22 degrees after which they declined to very low levels at 40 degrees. Yields of testosterone and 11-ketotestosterone glucuronides while showing a peak at 22 degrees declined much more slowly at higher temperatures than did those of the free steroids. In the absence of pituitary stimulation, levels of all steroids were below the limits of detection. Plasma levels of testosterone (15.3 +/- 1.5 ng/ml), 11-ketotestosterone (5.3 +/- 2.7 ng/ml), 11 beta-hydroxytestosterone (5.5 +/- 2.6 ng/ml), and their glucuromides (1.5 +/- 0.5, 0.14 +/- 0.1, and 1.5 +/- 0.5 ng/ml, respectively) were measured in fish held at 25 degrees. A rapid conchromatographic method for the assay of the three free steroids is described and the results are shown to be comparable to those obtained after chromatography.