Two groups of railway workers (n = 16 and n = 17) have been followed on their place of work during a period of shifts between day and night work. Catecholamine excretion in the urine and blood levels of lipids, glucose, uric acid, potassium and calcium were followed during the different phases of shift work. Dramatic fluctuations were noted in the diurnal pattern of catecholamine excretion during and after night work. Significant elevations in the serum levels of cholesterol, glucose, uric acid and potassium were observed during the first week after a night shift, and these changes could not be explained on the basis of shifts in the diurnal pattern or changes in dietary or other habits.