The serum concentration of alpha 1-acid glycoprotein (AAG) was determined hourly for 25 h in male and female volunteers. Subjects were carefully selected to control for age, weight, health status, smoking history, medication use, and, for females, estrus cycle and oral contraceptive use. Studies were performed under conditions closely controlled for sleep, daytime environment, and diet. All mean concentrations were within the range reported as normal, but the deviation around individual means varied from 6.4 to 48.9%. Males had higher mean AAG concentrations than females, 59.9 versus 52.4 mg/dL, but the difference was not statistically significant. Studies which compare changes in AAG concentration or the degree of basic drug protein binding over time within individuals or between groups should consider intrasubject variability in AAG for accurate interpretation of results.