Triphenyltin alters androgen metabolism in the sea urchin Paracentrotus lividus. Androgen metabolism (androstenedione and testosterone) has been assessed in the digestive tube and gonads of the echinoderm Paracentrotus lividus exposed to different concentrations of the biocide triphenyltin (TPT) in a semi-static water regime for 4 weeks. Key enzymatic activities involved in both synthesis and metabolism of androgens, namely 17beta-hydroxysteroid dehydrogenases (17beta-HSDs), 3beta-HSDs, 5alpha-reductases, P450-aromatase, palmitoyl-CoA:testosterone acyltransferases (ATAT) and testosterone sulfotransferases (SULT), were investigated in digestive tube and/or gonads of control and TPT-exposed specimens in an attempt to see whether androgen metabolism was altered by exposure. In agreement with previous data for vertebrates, exposure to TPT led to a concentration dependent decrease of P450-aromatase that was statistically significant at the highest TPT concentration tested (225ng/L). Additionally, increased metabolism of testosterone to form dihydrotestosterone (DHT) and 5alpha-androstane-3beta,17beta-diol was observed, suggesting increased 5alpha-reductase activity in the gonads of TPT-exposed individuals. Interestingly, exposure to TPT induced testosterone conjugating activities in organisms exposed to medium (SULT) and high (ATAT and SULT) TPT concentrations. Despite the changes of androgen metabolizing enzymes, testosterone levels in gonads remained rather stable. In contrast, an increase in testosterone and a concomitant decrease in estradiol were observed in the coelomic fluid of TPT-exposed organisms. Overall, the data indicate the ability of TPT to modulate androgen metabolism and circulating steroid levels in P. lividus and suggest the existence of regulatory mechanisms to maintain stable endogenous levels of testosterone in gonads. This study also contributes to a better knowledge of echinoderm endocrinology.