Fractional [H-3]acetylcholine (ACh) release and regulation of release process by muscarinic receptors were studied in corpus striatum of young and aged rat brains. [H-3] Quinuclidinyl benzilate (QNB) binding and carbachol stimulated phosphoinositide turnover, on the other hand, were compared in striatal, hippocampal and cortical tissues. High potassium (10 mM)-induced fractional [H-3]ACh release from striatal slices was reduced by aging. Although inhibition of acetylcholinesterase with eserine (20 mu M) significantly decreased stimulation-induced fractional [H-3]ACh release in two groups of rats, this inhibition slightly lessened with aging. Incubation of striatal slices with muscarinic antagonists reversed eserine-induced inhibition in fractional [H-3]ACh release with a similar order of potency (atropine = 4-DAMP > AF-DX 116 > pirenzepine) in young and aged rat striatum, but age-induced difference in stimulated ACh release was not abolish by muscarinic antagonists. These results suggested that fractional [H-3]ACh release from striatum of both age groups is modulated mainly by M-3, muscarinic receptor subtype. Although both muscarinic receptor density and labeling of inositol lipids with [myo-H-3]inositol decreased with aging, carbachol-stimulated [H-3]myo inositol-1-fosfat (IF1) accumulation was found similar in striatal, cortical and hippocampal slices.