Release of endogenous catecholamines (CA) by electrical nerve stimulation (NS) was studied in the isolated perfused spleen of the Atlantic cod, Gadus morhua. An HPLC-system for the analysis of endogenously released CA is described. Cocaine (COC) was used to block neuronal re-uptake of endogenous CA released by NS. Splanchnic NS at frequencies of 1-40 Hz for 20 s resulted in release of noradrenaline (NA) and adrenaline (A) with a maximal total overflow at 20 Hz for both amines. The release of CA was frequency-dependent. COC (0.1 mmol.l-1) increased NS-evoked (40 Hz) overflow of NA and A by 4.8 and 2.2 times, respectively. and reduced the overflow of dihydroxyphenylglycol (DOPEG) to spontaneous efflux levels or less. It can be concluded that the HPLC-technique used was adequate for measurement of NS-evoked release of endogenous CA and DOPEG from the isolated perfused cod spleen, and the model presented can therefore be used when studying adrenergic mechanisms in fish spleen.