Based on the results of studies on intracellular signaling in platelets of schizophrenics, an imbalance of the second messenger system is proposed: Diacylglycerol (DG), which activates protein kinase C (PKC), was increased, while adenylate cyclase (AC)-cAMP function was decreased. It is proposed that the increased DG/PKC function may entail a decrease in inositol 1,4,5-trisphosphate (IP3)/Ca2+ function and lowering of phosphoinositide turnover. If such a pathological intracellular signaling takes place in the brain, it may cause a distorted balance of protein activation via phosphorylation in neurons, resulting in some of the deficits of schizophrenia. Neuroleptics have been reported to antagonize the above-mentioned pathological processes of intracellular signaling. The imbalance hypothesis of the DG/PKC pathway and AC-cAMP pathway is not inconsistent with the dopamine hypothesis of schizophrenia, because dopamine receptor stimulation is indirectly related to reduction in IP3/Ca2+ function and lowering of phosphoinositide metabolism.