- 1. This study examined the saturation binding of tritiated gamma-aminobutyric acid [( 3H]GABA) and [3H]diazepam in brain membranes from young (3 month-old) and aged (21-23 month-old) Long Evans male rats killed at two time points in the 24-hour cycle. 2. The daytime density of low-affinity GABA binding sites was significantly (p less than 0.05) lower in cortical membranes from aged animals. There were no differences between young and old rats in low-affinity GABA binding at night, or in high-affinity GABA binding at either time point. 3. Diazepam binding was significantly lower in the brains of aged animals killed during the daytime. There was no differences at night, when diazepam binding in young animals declined to match that of aged animals. 4. There were no differences in the affinities of either GABA or diazepam binding sites. 5. These findings indicate that sampling time significantly influences age-associated changes in the densities of low-affinity GABA and diazepam binding sites. Therefore, the effects of age on brain receptor binding parameters should be measured at several points in the 24-hour light/dark cycle in order to control for possible age-related changes in binding rhythmicity.