Abstract
1. 1. The innervation of the heart of the trout ( Salmo trutta and S. iredius) has been studied with fluorescent histochemical and physiological methods. 2. 2. Catecholamine-containing nerves were revealed by the histochemical technique in the sinus venosus, atrium and ventricle. 3. 3. Stimulation of the vagus nerve or the ductus Cuvieri (especially at high-pulse frequency and duration) produced inhibition of the heart, which was blocked by atropine. After atropine treatment an excitatory response was revealed, which was blocked by guanethidine, bretylium or pronethalol. 4. 4. Stimulation of the vagus nerve or ductus Cuvieri at low-pulse frequency (less than 4 p/sec) and short duration produced excitatory effects even in preparations not treated with atropine. 5. 5. It is suggested that the cardiac branch of the vagus nerve contains both inhibitory cholinergic and excitatory adrenergic nerves.
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