The synthesis and central nervous system activity of a number of cinnolinylisobenzofuranones are described. The compounds are prepared in moderate yield by the reaction of the sodium salt of a cinnolin-4-one with the appropriate 3-bromophthalide. 1H, 13C n.m.r., and infrared spectral evidence for the structure of the compounds is presented. The compounds cause a loss of muscular control in mice.