1. Old rats showed a significant decrease in the number of muscarinic M(1) receptors and a significant increase in membrane microviscosity in the striatum and hippocampus as compared to young animals. In contrast, no significant changes in the density of muscarinic M(2) receptors were observed with aging. 2. Chronic treatment of aged rats with L-alpha-glycerylphosphorylcholine (L-alpha-GPC) restored the number of M(1) receptors to levels found in the striatum and hippocampus from young animals. The same treatment to aged rats partially restored membrane microviscosity in both regions studied and hence increased membrane fluidity. 3. None of the major metabolites of L-alpha-GPC (choline, glycerophosphate or phosphorylcholine) was able to restore the number of striatal and hippocampal M(1) sites and membrane microviscosity of aged rats, neither did any of these treatments (including treatment with L-alpha-GPC) modify the level of M(1) receptors and microviscosity values in young rats.