The toxicity of alcohol in relation to blood alcohol concentration (BAC) and body water was determined in young (31-36 days old) and adult (100-150 days old) male Long-Evans rats (groups of 5-15 rats each) after i.p. injections of 3, 4 or 5 g of alcohol/kg body wt as a 10% (wt/vol) solution. BACS were determined from tail blood samples taken 15, 90 and 240 min postinjection; body water was measured by the tritiated-water dilution technique. The younger rats had lower BACS than the adult rats at all doses and postinjection intervals (P < .05); they also had a higher body water content (P < .01). While the young and adult rats did not differ in the rate at which righting reflex and ability to stand were lost, the young rats showed significantly faster recovery (P < .01) of the righting reflex, reflexic movement and locomotor ability. These age-dependent differences increased as alcohol dosage increased, and may be due to differences in CNS maturation, BACS or both. Age is a significant factor in alcoholism research in rats.