δ9-THC in doses of 0.25, 0.5 or 1.0 mg/kg was administered to pairs of squirrel monkeys in a water competition test. After stable baseline measures for the pairs were obtained, repeated testing of pairs followed the sequence of orally treating the dominant member, the submissive member, and both members of the pairs. Total competition behaviors decreased when both members received 1 mg/kg, but increased when either the submissive member or both members of low competitive pairs received 0.25 mg/kg. Noncompetitive social behaviors increased when both animals were drugged, an effect which was maximal at 1.0 mg/kg. δ9-THC produced the most salient effects when both animals were drugged and demonstrated a biphasic dose effect on competition.