The tonical cholinergic and adrenergic influence on the heart rate was investigated in vivo in seven species of marine teleosts (pollack, Pollachius pollachius; cuckoo wrasse, Labrus mixtus; ballan wrasse, Labrus berggylta; five-bearded rockling, Ciliata mustela; tadpole fish, Raniceps raninus; eel-pout, Zoarces viviparus and short-spined sea scorpion, Myoxocephalus scor pius) during rest and, in two of the species (P. pollachius and L. mixtus), also during moderate swimming exercise in a Blazka-type swim tunnel. Ventral aortic blood pressure and heart rate were recorded via a catheter implanted in an afferent branchial artery, and the influence of the cholinergic and adrenergic tonus on the heart rate was assessed by injection of atropine and sotalol respectively. During rest the adrenergic tonus was higher than the cholinergic tonus in all species except L. berggylta, where the reverse was true. In P. pollachius and L. mixtus, exercise appeared to produce a lowering of the cholinergic tonus on the heart and, possibly, a slight increase of the adrenergic tonus. The nature of the adrenergic tonus (humoral or neural) is not clear, but the low plasma concentrations of catecholamines both during rest and exercise could be interpreted in favour of a mainly neural adrenergic tonus on the teleost heart. These experiments are compatible with the view that both a cholinergic inhibitory tonus and an adrenergic excitatory tonus are general features in the control of the teleost heart in vivo, both at rest and during moderate swimming exercise.