The effects of alcohol on blastocyst implantation and fecundity were determined. Pregnant rats (insemination = day 1) received vehicle only (water), or alcohol (2 or 4 g/kg body weight) daily by feeding tube on days 1-4 and the time of implantation determined on day 5 or fecundity assessed on day 19. Implantation sites were rendered visible by the blue-reaction; fecundity was expressed as the ratio of live fetuses to total implantations. The time course of blastocyst implantation was advanced by alcohol (2 g/kg dose). At 1000 hr, 1.9 vs. 0.9 blastocysts had implanted in treated vs. controls. By 1200 hr, 66 vs. 17% of blastocysts had implanted and by 1500 hr 90 vs. 52% had implanted in the 2 g/kg dose group vs. controls. Blastocyst implantation was not consistently advanced by the 4 g/kg dose of alcohol. The total number of blastocysts ultimately implanting did not differ between groups (control: 7.0 +/- 0.9; 2 g: 7.3 +/- 0.9; 4 g: 7.1 +/- 0.5 sites/cornu at 1800 hr). Fecundity did not differ markedly between control and the 2 g/kg dose group (97 vs. 96%, respectively), but was reduced in the 4 g/kg dose group (58%). The results indicate that daily administration of alcohol (2 g/kg body weight) during the preimplantation period (days 1-4) advances the time course of blastocyst implantation without reducing fecundity.