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Summary. Neuropeptide Y‐like immunoreactivity (NPY‐LI) and catecholamine concentrations in plasma were analysed during and after 60 min of physical exercise at a work load corresponding to 70% of individual maximal oxygen uptake in nine healthy men of average physical fitness. Systemic plasma NPY‐LI increased progressively from 18±3 to 81±19 pmolx1‐1 in parallel with a 10‐fold increase in noradrenaline (NA) concentration. The increase in plasma NPY‐LI during exercise and the decrease after completion of exercise were much slower than the corresponding changes in NA concentration. This difference is probably related to a slower diffusion of NPY into systemic circulation after release, as well as to a longer half‐life of NPY than of NA in plasma. Reversed phase HPLC and sephadex G‐50 gel‐filtration chromatography revealed that the main component of NPY‐LI in plasma during exercise eluted in a similar position as synthetic human NPY. During exercise plasma NPY‐LI correlated well with the plasma concentration of NA (r=0·80), but not with that of adrenaline (ADR), suggesting a neuronal origin of NPY. The self‐ratings of perceived exertion (RPE) were well correlated with the plasma concentrations of both NPY‐LI and NA. No clear‐cut veno‐arterial concentration difference was observed for NPY‐LI. Isometric handgrip and orthostatic test doubled plasma NA concentrations but did not cause any increase in plasma NPY‐LI. No change in plasma tachykinin‐like immunoreactivity was detected during exercise. The present data suggest that NPY is released together with NA during strong, but probably not during mild, sympathetic activation under physiological conditions in man.