Levels of progesterone (P4), testosterone (T) and estradiol (E2) indicated significant variation among individual echinoids during the annual cycle, reflecting generally the variation in gamete development that can be observed among individuals. Testosterone and E2 levels in both the ovaries and testes were higher during the period of gonadal growth. Levels of all steroids were greatly reduced compared to those levels reported for asteroids. Differences in the levels of P4, T, and estrogens between asteroids and Lytechinus variegatus may be related to differences in gonad morphology and nutrient storage capacity between asteroids and echinoids. It was hypothesized that the low levels of steroids detected in L. variegatus reflect paracrine-like mechanisms in cell signaling as compared to endocrine-like mechanisms proposed to be involved in regulating gonad function in asteroids. Both the ovaries and testes of L. variegatus had the capacity to synthesize T and a variety of 5α-reduced androgens including 5α-androstane-3β,17β-diol and 5α-androstane-3α,17β-diol (5α-adiols) from androstenedione (AD) in 8 h. Estrogen synthesis was not detected. The sex-specific pattern of accumulation of 5α-adiols in the ovaries and testes suggests that the 5α-adiols may affect processes related to reproduction in L. variegatus. Copyright (C) 2000 Elsevier Science Inc.