Doxylamine succinate was administered as an admixture in the feed to male and female Fischer 344 rats for either 14 or 90 days. The 14-day study included dose levels of 0, 100, 250, 500, 1000, or 2000 ppm doxylamine. Except for a 7% decrease in final body weight in female rats in the 2000 ppm group, there were no significant clinical observations made in the 14-day study. Microscopic lesions judged to be treatment-related were limited to cytoplasmic vacuolization in the livers. The lesions were more numerous in the higher dose groups of males and present only in the 2000 ppm group of females. Dose levels of 0, 162, 405, 1012, 2530, and 6325 ppm doxylamine were administered in the 90-day study. There were no deaths during the study. Final body weights were decreased 13.3% in males of the 6325 ppm group and 5.2, 10.1, and 14.4% in females in the 1012, 2530, and 6325 ppm groups, respectively. Liver/brain weight ratios were increased in all treated male groups and in the two highest dose groups of females. Other organ weight changes were decreases and believed to result from general reduction in weight gain in those groups where the decreases occurred. Treatment-related histological changes were identified in the liver and parotid salivary gland. Cytoplasmic vacuolization or fatty change of the liver was found in all groups of males but was more severe in the higher dose groups. In females, these liver lesions were observed only in the two highest dose groups. A dose-related change in the parotid salivary gland, consisting of cytomegaly with basophilic and coarsely granular or vacuolated cytoplasm, was observed.