I.p. injections of epinephrine (0.05 .mu.g/g) elicited characteristic changes in the abundance of circulating leukocytes at selected time intervals post-injection in Colisa. Leukocytosis was evident at 15, 27 and 75 min and a tendency towards leukocytosis was observed at 123 and 267 min; at 3, 171, 219, 315 and 363 min and at 12 and 24 h, the total leukocyte counts for the experimentals and controls were not significantly different. No significant changes were observed in the abundance of circulating erythrocytes or thrombocytes. Leukocyte sequence elicited by epinephrine was also apparent in Colisa which were exposed to a temperature of 2.degree. C for 1 min. Pretreatment with phenoxybenzamine (3.0 mg/l aquarium water), an .alpha.-adrenergic antagonist, abolished the cold-shock leukocytic phases, except at 27 min, when leukocytosis was unaffected. Exogenous norepinephrine (2 .mu.g/g) and isoproterenol (0.1 .mu.g/g) failed to elicit any significant change in the number of circulating leukocytes, erythrocytes or thrombocytes. Clearly, adrenergic mechanisms are involved in the cold-shock leukocyte stress sequence. The results suggest .alpha.-activating role of epinephrine during the leukocyte stress syndrome in Colisa; apparently, epinephrine is leukocytic.