Eprozinol inhibits bronchoconstriction by an action on histamine H1 receptors and has been used in the treatment of asthma and bronchitis. No significant changes in cAMP and cGMP levels are observed in guinea pig trachea, with eprozinol or isoprenaline, at doses capable of inducing relaxation. Eprozinol is only a very weak phosphodiesterase inhibitor, at large concentrations. The anti-bronchoconstrictor activities of eprozinol and isoprenaline with regard to histamine are directly additive and show absolutely no interference with one another. Propranolol is without effect on in vivo anti-bronchoconstrictor activity of eprozinol on tracheal musculature. It is concluded, that the mechanisms brought into play by eprozinol to exert anti-bronchoconstrictor and bronchorelaxant activity, are completely independent of the adrenergic system. In a retrospective study of 199 cases of accidental or intentional acute poisoning with eprozinol, eprazinone and zipeprol, collected at the Poison Control Center were seven cases of seizures, all after ingestion of eight times the therapeutic dose. They resolved rapidly and without recurrence with symptomatic treatment.