Urinary levels of free adrenaline and noradrenaline were measured in two groups of healthy male industrial workers exposed to alternate four-day periods of working conditions with and without time stress, to test the hypothesis that the sympathetic nervous system is overactivated by occupational stress. Thirty confectionary workers alternated piece-work (payment by results) and work with a fixed daily wage while 30 metal workers alternated work on an assembly line with work off it. Under time stress urinary free adrenaline was 450 per cent and noradrenaline 230 per cent of the levels for similar work without time stress but involving equal oxygen consumption. These differences were statistically highly significant and they persisted on retesting after six months of alternating work regimens. They support the concept that occupational stress in industrial workers influences the adrenosympathetic system and they indicate a possible method for assessing the effects of high levels of sympathetic activity on the aetiology of ischaemic heart disease.