The effect of 9 ppm halothane, a subanesthetic concentration, was studied on the reproductive ability and pregnancy outcome in white rats. Pregnant females were exposed for 4 h per day during the whole gestation period. Male animals were subjected to halothane inhalation for 4 h daily, 5 days per week for 6 or 8 months. In the exposed females higher rates were established for early interruption of pregnancy (all implants dead) and for embryonic death in the later periods of intrauterine life compared to controls. Deciduomata were found in 22.22% (P less than 0.05) of pregnant treated females. Control females mated to exposed males showed a higher incidence of preimplantation loss (36.36% (P less than 0.05) of those mated with males exposed for 6 months had deciduomata). Decreased fertility in males, expressed particularly in the 8-month treated group, was also demonstrated (13.33% inseminated females, compared to 35% in the control group).