The serum concentration of testosterone was estimated from a population of wild lungfish over 6-7 years of sampling. Male lungfish were found to have high circulating levels of testosterone (~50 ng/ml) which varied seasonally and could be correlated with spermatogenesis as judged by testis histology. Incubation of testis tissue slices with [3H]progesterone, [3H]17- hydroxyprogesterone, or [3H]testosterone confirmed that testosterone is the major androgen in Neoceratodus. Not even trace amounts of 11-keto- or 11β- hydroxytestosterone or 5α-dihydrotestosterone could be identified by TLC separations. There was little or no conjugation of steroids by the testes, except during the spawning season, when glucuronides of androstenedione and testosterone were produced.