A subacute oral toxicity study of acetaldehyde and formaldehyde was carried out in rats. Groups of ten male and ten female 5-wk-old rats received one of the aldehydes in the drinking-water for a period of 4 wk, acetaldehyde being given at dose levels of 25, 125 and 675 mg/kg body weight/day and formaldehyde at dose levels of 5, 25 and 125 mg/kg body weight/day. A group of 20 males and 20 females served as controls and received unsupplemented drinking-water ad lib. An additional group of ten males and ten females was given unsupplemented drinking-water in an amount equal to the amount of liquid consumed by the group given the top dose of formaldehyde. Food and liquid intake were decreased in the groups on the top dose of both acetaldehyde and formaldehyde. Hyperkeratosis of the forestomach, observed only in the top-dose rats, was the only adverse effect of acetaldehyde detected. Effects of formaldehyde, also observed only in the top-dose group, were yellow discoloration of the fur, decreased protein and albumin levels in the blood plasma, thickening of the limiting ridge and hyperkeratosis in the forestomach, and focal gastritis in the glandular stomach. It was concluded that in this study the no-observed-adverse-effect levels of acetaldehyde and formaldehyde were 125 and 25 mg/kg body weight/day, respectively.