Mild depolarisation (20 mM KCl) synergistically enhances the ability of a muscarinic agonist to activate phosphoinositide turnover and to elevate inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in cerebellar granule cells in primary culture. The effects of lithium on this intense stimulation of phosphoinositide turnover was studied. Lithium causes depletion of cytoplasmic inositol and phosphoinositides, which results in the inhibition of phosphoinositide turnover within 15 min and the return of Ins(1,4,5)P3 to basal levels at this time. This inhibition could not be reversed by culturing and preincubating cerebellar granule cells in concentrations of inositol similar to those detected in the CSF. Inositol concentrations substantially in excess of those in the CSF not only reversed the effects of lithium on stimulated Ins(1,4,5)P3 levels, but significantly enhanced this level in comparison with stimulation in the absence of lithium. sn-1,2-Diacylglycerol elevation during stimulated phosphoinositide turnover was also disrupted by lithium, but in contrast to Ins(1,4,5)3, the presence of lithium resulted in a transient enhancement of the elevation evoked by carbachol plus mild KCl depolarisation, which was reversed by 500 μM inositol, but not by 200 μM inositol. The implications of these phenomena in relation to the mechanism of action of lithium in the treatment of manic depression are discussed.