Hyperactive boys on methlyphenidate (Ritalin), hyperactive boys on placebo, and comparison boys were observed in quasi‐naturalistic classroom settings. Ambient stimulation (quietversusnoisy conditions) and source of regulation (self‐pacedversusother‐paced activities) were varied in a 2 × 2 design. Compared to their peers, hyperactive boys on placebo showed lower rates of task attention and higher rates of gross motor movement, regular and negative verbalization, noise‐making, physical contact, social initiation, disruption, and acts that were perceived as energetic, inappropriate, or unexpected. Self‐paced activities resulted in increased rates of verbalization, social initiation, and high‐energy episodes. High ambient noise levels reduced task attention and increased the rates of many other behaviors including verbalization, physical contact, gross motor movement, and high‐energy acts. Medication‐by‐situation interactions emerged for both classroom dimensions, with hyperactive boys on placebo being readily distinguishable from their peers under some classroom conditions and indistinguishable under other conditions. Moderate relationships were found between teacher ratings and many individual behavior categories. Discussion focused on (a) the merits and limitations of a social ecological research perspective, and (b) the implications of these findings for the design of inter