The content of acetylcholine and histamine in the effluent of isolated, vagally innerved guinea-pig auricles was determined. Spontaneous or stimulation-induced overflow of acetylcholine was detected only in the presence of acetylcholinesterase inhibitors. The histamine overflow was measurable also in the absence of inhibition of cholinesterase, and neatly increased during vagal stimulation. The vagally evoked histamine overflow was blocked by atropine and potentiated by eserine. The stimulation-induced histamine overflow in the effluent is discussed, presumably assuming that acetylcholine may release histamine from cardiac histamine stores.