Reported were results of the first year of a 3-year physiological study of the hyperkinetic child. The male subjects were 6 to 9 years of age, attending school, without sensory defects, 80 or above in Wechsler Intelligence Scale for Children Full Scale, off medication for 3 months prior to testing, and diagnosed as hyperactive. Electroencephalograph and evoked cortical measures were made for 31 hyperkinetic children and 21 normal controls in order to predict clinical response to stimulant medication. Experimental design included a structured interview, teacher and parent rating scales, medical evaluation, psychological testing, watching a video taped cartoon while taking cortical measures at beginning and end of 3-week period, and Ritalin and placebo treatments. Overall results indicated existence of a fundamental physiological difference between children responding well and poorly to stimulant medication. Low central nervous system arousal and good clinical response to stimulant treatment were found to characterize one group, while high central nervous system arousal and poor response to stimulant treatment were found to typify the other group. (CB)