The effect of isoprenaline and three different beta-adrenergic blocking agents on blood eosinophilia was evaluated in the rat. Eosinopenic effect of isoprenaline was antagonized by intraperitoneal pretreatment with propranolol and by high but not low doses of practolol. Sotalol, a beta-blocker which in contrast to propranolol does not penetrate the blood-brain barrier, did not affect the eosinophil count when injected intraperitoneally, but produced a marked increase in number of circulating eosinophils following intracerebroventricular injection. Intraperitoneal administration of isoprenaline and propranolol as well as intracerebroventricular injection of sotalol did not affect eosinophil count in adrenalectomized or hypophysectomized rats. These results suggest that beta-adrenergic agents regulate the number of circulating eosinophils through a central mechanism and that presence of both hypophysis and adrenal glands is required for this action.