1. Male Swiss mice were treated systemically with beta-phenylethylamine (PEA, 25-150 mg/kg), and observed in isolation or in groups of five. The effects of PEA were scored on 13 separate components of behaviour which were produced or increased by the drug. 2. PEA at a dose of 25 mg/kg depressed activity and caused sedation, but at 50 mg/kg produced a brief stimulation of activity. At higher dose levels (75-150 mg/kg) the compound induced a biphasic stimulation of activity which was associated with the development of two distinct groups of stereotyped activities. 3. Group testing significantly antagonized early phase stereotypy (forepaw padding, headweaving, compulsive grooming) but had no effect on, or potentiated, late phase stereotypy (rearing, licking). In addition grouped mice were more active and hyperreactive than isolated mice. 4. The results are briefly discussed in the light of the possibility of a catecholamine/indoleamine interaction in the control of the hyperactivity syndrome induced by PEA in the mouse.