Atracurium besylate, a recently developed, intermediate-duration acting, neuromuscular-blocking agent, was given to 15 halothane-anesthetized ponies to produce surgical relaxation (95% to 99% reduction of hoof twitch). All 15 ponies were given 3 injections; 8 of the 15 ponies were given 2 additional injections. Initial dosage of 0.11 +/- 0.01 mg/kg (mean +/- SD) and all subsequent injections of 0.052 mg/kg produced desired relaxation. Paralysis phase (maximum twitch reduction to 10% twitch recovery) lasted 24 +/- 5 minutes for the initial injection. Paralysis from subsequent injections lasted for a slightly shorter time. Recovery phase (10% to 75% twitch recovery) was similar for all injections (initial and repeated) and lasted approximately 11 minutes. Cardiovascular side effects were not seen. Reversal of effects of atracurium with administration of 0.5 mg of edrophonium/kg was achieved when the evoked digital extensor tension (twitch height) had returned to 95% of base line after the last atracurium injection. Edrophonium caused systolic blood pressure to increase 121% +/- 7% of base-line pressure, which was 133 +/- 18 mm of Hg. Heart rate changed to 93% +/- 9% of base line after edrophonium was given, which was 49 +/- 7 beat/min, but this change did not occur until after the blood pressure increased. Recovery to standing was smooth and strong. Five ponies stood on their first attempt to rise, 5 on the 2nd attempt, 2 on the 3rd, and 1 on the 4th. Seven ponies stood within 30 minutes after transportation to the recovery stall, 7 within an hour.(ABSTRACT TRUNCATED AT 250 WORDS)