Abstract: Group turnout provides important socializing opportunities for horses, particularly those that are primarily stalled. A high percentage of equine injuries occur during group turnout, which could partly be due to the physical constraints of fencing. To investigate appropriate paddock sizes for group turnouts, horses (n = 12) from a single herd were divided into groups of 4, stalled for 24 hours, and then turned out for 1 hour into one of three differently sized pens: 342, 263, and 184 m2 per horse. Groups rotated through pens across 3 days, receiving one treatment per day. Blood was sampled for cortisol concentrations at 08:00 hours each morning, and then at 15 and 60 minutes into the turn out sessions, and 60 minutes after return to individual stalls. Groups rotated through three turnout times: 09:00, 12:00, 14:00 hours. Counts of agonistic behaviors (chasing, contact biting, and kicking) and low-level threats (pinned ears, tail swishing, bite and kick threats) were recorded. When turned out in pens that provided 342 m2 per horse, horses exhibited reduced plasma cortisol concentrations by 15 minutes after turnout and at 1 hour after return to their stalls (P < .05). Horses in pens providing 184 m2 per horse exhibited greater agonistic (P < .001) and low-level threat (P < .01) behaviors than horses in larger pens. These data provide insight into appropriate pen sizes for horses from established herds. Providing at least 342 m2 per horse may reduce the chance of injury in horses accustomed to group turnout.