Microviscosity of membranes prepared from striata and cortex of 3 or 30-month-old rats was measured by fluorescence polarization and electron spin resonance. The viscosity of the hydrophobic core of the lipid bilayer was significantly increased in striatal but not in cortical membranes of old rats. In old rats the incorporation of methyl groups into phosphatidylcholine was significantly lower than in young rats. beta-Adrenergic specific binding sites were reduced in striata, cortex and pineal gland of old rats. In the striata of these animals, [3H] spiperone specific binding sites and dopamine-stimulated adenylate cyclase were also low. A chronic treatment of old rats with S-adenosyl-L-methionine, the cofactor in phospholipid methylation, lowered microviscosity to normal values in striata but not in the cortex. In the same old rats, the beta-receptor in striata and pineal gland returned towards juvenile values; in the cortex, the modification was in the same direction but did not reach significance. Neither putative dopaminergic receptors nor the adenylate cyclase age-dependent decrease was modified by treatment with S-adenosyl-L-methionine.