A biphasic maximal inhibition was present at 1 microM concentrations when the retinoids VITA, TIG and ROA were added for 16 h (52, 58 and 57%, respectively; P < 0.01 by one-way analysis of variance). In contrast, the addition of the three retinoids at 1 microM concentrations for 16 h had no significant effect on HCG secretion by placental explants of 11-13 weeks gestational age.