Research Allergological Laboratory of the USSR Academy of Medical Sciences and Laboratory of Pharmacology, S. Ordzhonikidze Research Chemico-Pharmaceutical Institute, Moscow. Among the tested new antihistaminic drugs (quinuclidine derivatives) quinuclidyl-3-(O-tolyl) carbinol possessed histamine releasing action (HRA) on the isolated rat mast cells. In used concentrations (up to 0.4 mmol) all phenothiazines (promethazine, phenethazine, chlorpromazine, methylene blue) had HRA. There was no correlation between the HRA and the antihistaminic activity of the tested drugs. Histamine release induced by antihistaminic drugs and a steep dose-response curve, was produced at low temperature and was not inhibited under conditions of inhibition of energy-dependent stage of 48/80-induced histamine release. It was concluded that the tested antihistaminic drugs which had HRA were non-selective histamine releasers.