This research is primarily concerned with the effects of chronic low doses of carbon monoxide on fetal development. Carbon monoxide was administered daily by inhalation to female Swiss Webster mice from the beginning of gestation until term. Daily weights were recorded and carbon monoxide blood levels determined every 4 days. The number of offspring in each litter was recorded. At weaning, two males and two females from each litter were randomly picked for maze running studies. When the mice were 6 weeks old, they were tested daily, ten trials per day, in the maze until learning had occurred. The number of days required to learn the maze and the number of incorrect trials were recorded. While there was no significant increase in the number of days needed to learn the maze, there was a significant increase in the number of errors made by the experimental group during this time. This indicated that an increased effort was needed to learn the maze.