1. Plasma catecholamine, haemodynamic and metabolic responses to sustained isometric exercise were studied in eight healthy subjects, who maintained handgrip at the 30% level of maximal voluntary contraction (MVC) for as long as possible. 2. The sustained handgrip was accompanied by a significant increase in plasma noradrenaline (NA) and adrenaline (A) concentrations. 3. The increase in plasma NA during handgrip was greater than that associated with heavy dynamic work involving large muscle groups. 4. The results suggest that the known haemodynamic responses to static effort are related to a powerful activation of the adrenergic system, which may result from a reflex mechanism initiated in the exercising muscles.