The ability of the antarctic pteropod Clione antarctica (Mollusca: Gastropoda) to synthesize sex steroids from radiolabeled steroid precursors was examined in individuals sampled near the end of their observed reproductive season from McMurdo Sound, Antarctica. The precursors, 3H-progesterone and 3H-androstenedione, were absorbed and subsequently metabolized to produce several progestin and androgen metabolites, including 5α-pregnane-3,20-dione, 3β-hydroxy-5α-pregnan-20-one, 3β-hydroxy-4-pregnen-20-one, 5α-androstane-3,17-dione, 3β-hydroxy-5α-androstan-17-one (androsterone), and 3β-hydroxy-5α-androstan-17-one (epiandrosterone). Synthesis or accumulation of testosterone and estradiol was not observed. At least 2 unidentified, water-soluble progestin metabolites were synthesized, as well as an unidentified esterified steroid. The metabolic accumulation of these steroidal compounds indicates that C. antarctica can produce potentially bioactive steroids. The activity of steroid metabolic enzymes in a polar species further indicates the universal capacity of invertebrates from various geographic regions to metabolize biologically relevant steroids.