A comparative approach was taken in this study to evaluate androgen (androstenedione and testosterone) metabolism in three invertebrate species: the gastropod Marisa cornuarietis, the amphipod Hyalella azteca, and the echinoderm Paracentrotus lividus. The existence of 17beta/3beta-hydroxysteroid dehydrogenase (HSD) and 5alpha-reductase catalyzed reactions was demonstrated in all three species. Androstenedione was primarily converted to 5alpha-androstanedione in M. cornuarietis, while it was primarily metabolized to testosterone in P. lividus and H. azteca. In addition, and consistent with vertebrate findings, tissue specific pathways and sexual dimorphism in androgen metabolism were observed. Namely, testosterone was metabolized to dihydrotestosterone in P. lividus gonads (via 5alpha-reductase), and metabolized to 4-androstene-3beta,17beta-diol in the digestive tube (via 3beta-hydroxysteroid dehydrogenase). Furthermore, the synthesis of 17beta-reduced metabolites of androstenedione (testosterone and dihydrotestosterone) was 3- to 4-fold higher in males of M. cornuarietis than in females. Organotin compounds, which have been shown to interfere with some aspects of androgen metabolism, had no major effect on testosterone metabolism in any of the three species. Fenarimol enhanced 5alpha-reductase-mediated catalysis in gonads of P. lividus. Overall, results demonstrate the ubiquity of some androgen biotransformation processes in invertebrates and reveals interphyla differences in androgen metabolic pathways, and different sensitivity of these pathways to some xenobiotics.