The present study examined the effects of tributyltin (TBT), polychlorinated biphenyls (PCBs), and mixtures of both chemicals on reproduction in Japanese medaka, Oryzias latipes. For 21 d we gave groups of medaka freeze-dried brine shrimp flakes contaminated with a mixture of either 0, 1, 5, or 25 microg TBT g(-1) plus 0 or 25 .micro PCBs g(-1). We measured the fecundity and fertility of the parent fish and assessed the deformity, hatchability, time-to-hatching, and swim-up failure rate of the next generation. Fertilization success in the third week of the administration period was significantly decreased by administration of 25 microg TBT g(-1) (77%) compared with the control group (87%). Both TBT and PCBs were transferred maternally into the eggs of the next generation, causing early life-stage toxicity. Administration of 1 microg TBT g (-1) was not toxic to embryological development, but abnormal eye development (i.e., small eyes or no eyes) occurred when TBT at the same concentration was mixed with PCBs (6.4%). Administration of TBT alone significantly decreased hatchability and increased swim-up failure, and administration of PCBs alone significantly increased time-to-hatching. Statistical analysis by two-way analysis of variance detected an interaction between TBT and PCBs in these three parameters. TBT induces abnormal development of the eyes, reduced hatchability, and increased swim-up failure, whereas PCBs delay time-to-hatching. Administration of mixtures of TBT and PCBs has more adverse effects on the developmental stage of medaka than does that of each chemical alone.