Levels of aggression, activity and performance were determined in 270 pigs (initial wt 29.8 kg) injected with amperozide (1.0 mg/kg i.m.), azaperone (2.2 mg/kg i.m.) or saline (.1 ml/kg i.m.) immediately prior to mixing. Pigs had ad libitum access to feed in pens of 15, and six pens were allotted to each treatment. Each pen was video-taped for 48 h after injection. Aggression was determined by continuous observation and summarized for each 2-h period. Injuries on the ears and shoulders of each pig were scored prior to injection and 1, 2, 3 and 7 d after treatment. Eating, drinking and lying were determined by scan sampling at 2-min intervals and summarized for each 2-h period. Weight gain, feed consumption and efficiency were determined for periods ending on d 3, 7, 14, 28, 42, 56, 70 and 84. Both drugs reduced total fighting from 309.8 min for saline to 190.7 and 189.6 min for amperozide- and azaperone-treated pens, respectively (P less than .01). Treatment differences in aggression and lying were evident during the initial 6 h only. Amperozide resulted in fewer fights involving two pigs (197.3/pen) than did azaperone (260.2/pen) or saline (298.3/pen) (P less than .05). Injuries to the ears (P less than .01) and total injuries (P less than .05) were less severe in amperozide-treated pigs than in pigs on the other treatments.(ABSTRACT TRUNCATED AT 250 WORDS)