Synbranchus marmoratus is a facultative air-breathing fish, which uses its buccal cavity as well as its gills for air-breathing. S. marmoratus shows a very pronounced tachycardia when it surfaces to air-breathe. An elevation of heart rate decreases cardiac filling time and therefore may cause a decline in stroke volume (V(S)), but this can be compensated for by an increase in venous tone to maintain stroke volume. Thus, the study on S. marmoratus was undertaken to investigate how stroke volume and venous function are affected during air-breathing. To this end we measured cardiac output (Q), heart rate (f(H)), central venous blood pressure (P(CV)), mean circulatory filling pressure (MCFP), and dorsal aortic blood pressures (P(DA)) in S. marmoratus. Measurements were performed in aerated water (P(O2)>130 mmHg), when the fish alternated between gill ventilation and prolonged periods of apnoeas, as well as during hypoxia (P(O2)<or=50 mmHg), when the fish changed from gill ventilation to air-breathing. Q increased significantly during gill ventilation compared to apnoea in aerated water through a significant increase in both f(H) and V(S). P(CV) and MCFP also increased significantly. During hypoxia, when the animals surface to ventilate air, we found a marked rise in f(H), P(CV), MCFP, Q and V(S), whereas P(DA) decreased significantly. Simultaneous increases in P(CV) and MCFP in aerated, as well as in hypoxic water, suggests that the venous system plays an important regulatory role for cardiac filling and V(S) in this species. In addition, we investigated adrenergic regulation of the venous system through bolus infusions of adrenergic agonists (adrenaline, phenylephrine and isoproterenol; 2 microg kg(-1)). Adrenaline and phenylephrine caused a marked rise in P(CV) and MCFP, whereas isoproterenol led to a marked decrease in P(CV), and tended to decrease MCFP. Thus, it is evident that stimulation of both alpha- and beta-adrenoreceptors affects venous tone in S. marmoratus.