In experiments on 2-, 10- and 22-month-old rats it was found that the Bmax values of dopamine (DA2), serotonin (5-HT1) and enkephalin (Enk) receptors as well as th( 5-HT level in the three brain regions (cortex, striatum and hypothalamus) decreased with age; the DA level in the brain cortex and striatum and the noradrenaline (NA) content in the brain cortex decreased, while the NA level in the striatum and the 5-hydroxyindolacetic acic (5-HIAA) level in the brain cortex and the striatum as well as the MAO-T and MAO-A activities in the three brain structures increased. It is suggested that these and other change: observed in brain neurotransmission are an important element in the neurochemical bases oï the age-related changes in behavior.