I investigated gonadal differentiation and steroidogenesis of sea lamprey, ' Petromyzon marinus'. Gonadal biopsy of large larvae was used to compare initial and subsequent assessments of morphology. Sex reversals, oocyte atresia and testis development from atypical gonads suggest that gonadal sex remains labile throughout most of the larval phase. The resultant increase in male abundance supports the argument that high lamprey density produces male larvae. I observed production of a potentially feminizing hormone (15[alpha]-hydroxyprogesterone) by gonads of larvae and juveniles. No synthesis of potentially masculinizng androgens was apparent, nor did larval gonads appear to metabolize androgen substrate. However, environmental stimuli are associated with larval masculinization. Larval gonads, therefore, may differentiate into ovaries by default and into testes via an environmental influence which induces masculinizing steroid synthesis/metabolism. Larvae about to metamorphose and metamorphosing and transformed lampreys metabolized androgen precursor which produced steroids possibly used in gonad development and/or metamorphosis.