It was shown that a marked therapeutic effect or its absence in the piracetam and nootropil treatment of children with different degrees of mental retardation is largely determined by the presence in patients' metabolism (before treatment) of certain combined changes in the general activity of lactate dehydrogenase (LDH) and its isoenzymic spectrum with regard to the ratio between aerobic and anaerobic subunits. Experimental studies on rats support the necessity of taking into account patients' metabolism when prescribing nootropic agents. It is suggested that considering LDH changes (along with the clinical condition of patients) may be of prognostic significance in the efficacy assessment of treatment with nootropic drugs.