The embryotoxic and teratogenic potential of sulfur dioxide (SO2) was evaluated in CF-1 and New Zealand rabbits exposed to SO2 alone or in combination with carbon monoxide (CO). The animals inhaled filtered room air (controls), SO2 (mice, 25 ppm; rabbits, 70 ppm), or SO2 plus CO (250 ppm) for 7 hr/day from days 6 through 15 (mice) and from days 6 through 18 (rabbits) of gestation. In both species, inhalation of SO2 resulted in slight toxicity in the dams and an increased incidence of minor skeletal variants among their offspring; exposure to the combination did not potentiate the increased incidence of these variants. A teratogenic effect was not discerned in either mice or rabbits exposed to SO2 alone or in combination with carbon monoxide, but the fetuses of mice exposed to the combination were significantly smaller than those exposed only to SO2.