The cod heart regulation has been investigated by anatomical studies, fluorescent histochemistry, nerve stimulation and drug effects on the perfused heart and electrically paced strips. The effect on the heart of catecholamine release from the head kidney has been studied in a perfused head kidney and heart preparation. Branches of the vagi and the fused 1st and 2nd spinal nerves innervate the heart. Specifically fluorescent fibres were found in these nerve branches, in the ducts of Cuvier, all parts of the heart, bulbus arteriosus and the ventral aorta. Weakly autofluorescent ganglion cells surrounded by specifically fluorescent varicose fibres were present in the walls of the sinus venosus. Vagal stimulation caused bradycardia altered after atropine to tachycardia. Spinal nerve and sympathetic chain stimulation also induced tachycardia, as did provoked catecholamine release from the head kidney. The tachycardia could be blocked by propranolol. Atropine increased and propranolol decreased basic heart rate and contraction force. Adrenergic drugs accelerated and acetylcholine retarded the perfused heart. A positive inotropic effect was obtained with adrenergic drugs on isolated heart strips, while acetylcholine produced a negative inotropic effect on atrial but not ventricular strips. It is concluded from this study that the cod heart has an inhibitory cholinergic nerve supply via the vagi and an excitatory adrenergic supply via both the vagi and the first spinal nerves. Sympathetically controlled release of catecholamines from the chromaffin tissue in the head kidney may also play an important role in the heart regulation.