Binding in 14 ligand membrane receptor pairs and choline acetyltransferase activity were studied at 4-hour intervals during a 24-hour cycle (12:12 light:dark). Cortex, hippocampus, cerebellum, striatum and brainstem were prepared from 3-, 12- and 24-month-old male rats. Cholinergic, α- and β-adrenergic, dopaminergic, serotonergic, gabaergic and opiate binding were determined. In the aged animals, the times of the binding maxima are no longer locked to the same time of the light:dark cycle, and the cycle phases themselves are shifted in comparison to those of the young and adult group. Cycle amplitudes were smallest in the 12-month-old rats, increasing significantly in the 24-month group. The age changes in the overall (24-hour) means fall into 4 different patterns, none of which shows a linear age-related decrease. This points out the great importance of including an adult group (12 months) in all ageing studies on small mammal