Fourteen children (12 infantile autism full syndrome present, 2 atypical pervasive developmental disorder) between 5 and 13 years of age participated in a double-blind placebo-controlled cross-over trial. Each child received 20 mg Org 2766 (synthetic analog of ACTH 4-9)/day during 4 weeks, or placebo in a randomly assigned sequence. Drug effects were monitored by ethological playroom observation and by Aberrant Behavior Checklist ratings by parents and teachers. Data of the playroom observation pointed to an activating influence of Org 2766, as revealed by a significant decrease of stereotypic behavior and significant increases in "change toys," "locomote," and "talk." Checklist ratings did not show significant changes. The clinical implications of these findings are discussed.