Abstract:Determinations were made of the plasma cyclic AMP level to examine its relationship with hyperkinesis (Werry‐Weiss‐Peters Activity Scale, WWPAS) and other features of mental disorders in 80 children, of whom 21 had early infantile autism, 15 hyperkinetic mental retardation, 12 minimal brain dysfunction and 32 Down's syndrome. In autistic and hyperkinetic mentally retarded children, the plasma cyclic AMP levels were higher than in normal children and were positively correlated with the WWPAS score. In children with minimal brain dysfunction, the plasma cyclic AMP level was significantly lower than in normal children and was not correlated with the WWPAS score. In children with Down's syndrome, the plasma cyclic AMP level was somewhat higher than in normal child