Mass spectrometric determinations confirmed that testosterone, 5alpha-androstan-17betaol-3-one, 5alpha-androstan-3alpha, 17beta-diol and 5alpha-androstan-3beta, 17beta-diol were present in venous effluent of in vitro perfused rabbit testes-epididymides. Testosterone, dihydrotestosterone, 3alpha-androstanediol and 3beta-androstanediol were secreted at 3.1 plus or minus 0.9, 0.7 plus or minus 0.2, 0.4 plus or minus 0.1 and 0.6 plus or minus 0.1 mug/h when testes-epididymides were perfused with an artificial medium containing 2.5 ng/ml NIH-LH-S17, ovine. Surprisingly, testosterone constituted only 64% of the total mass of the four androgens secreted. These results probably reflect in vivo androgen secretion since hourly collections of spermatic vein blood from anesthetized rabbits contained 2.7 plus or minus 0.9, 0.8 plus or minus 0.3, 0.15 plus or minus .04 and 0.5 plus or minus 0.2 mug of testosterone, dihydrotestosterone, 3alpha-androstanediol and 3beta-androstanediol, respectively. Once again testosterone constituted only 64% of the total mass of the four androgens.