Swiss, C57BL6 and CBA males were given 0.1 ml of 40% ethanol per mouse per day for three consecutive days, intraperitoneally. These males were mated with untreated virgin Swiss females employing a 4-day mating schedule and three consecutive matings were carried out. In another study, C57BL6 males were given an ascending gradient of 5% to 40% ethanol in drinking water for a total period of 11 weeks. These males were mated with C57BL6 females for 2 weeks. Females were dissected at mid-term pregnancy for the examination of uterine contents including total, live and dead implants. All the investigations comprised at least two or three independent experiments which were evaluated independently as well as after pooling the data. Swiss, C57BL6 and CBA males given 0.1 ml of 40% ethanol, intraperitoneally, gave no evidence of any significant increase in post-implantation lethality in the postmeiotic phase of spermatogenesis attributable to ethanol treatment. A moderate but significant reduction in mean total implants indicating pre-implantation losses was seen in Swiss but not in CBA mice. Prolonged feeding of ethanol up to 40% in drinking water failed to provide any evidence of dominant lethal mutations in C57BL6 males at the pre-implantation level and the post-implantation lethals were also not significantly higher than in controls. In Swiss mice, however, the mutagenic index based on both pre- and post-implantation lethality was consistently positive.