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Stress effects of 3-d (72-hr) continuous vigilance were studied for renal excretion of 17-oxycorticoids (17-KS), electrolytes (potassium and sodium), and liquid. The investigation involved 12 male subjects aged 23 to 36. Baseline parameters displayed diurnal variations with comparatively high day and comparatively low night values. During the 72-hr vigilance, significant and consistent (statistically fiducial) intensification of 17-KS and potassium excretion occurred in the night-time, i.e., in the period of naturally low values, but only in the first two sleepless nights. As for sodium and liquid excretion, stress-related reduction in these parameters was more consistent and distinct in the day-time, i.e., in the period of naturally high values, rather than in the nighttime. Therefore, the two factors that should be accounted for by investigators of continuous vigilance (and, maybe, other stresses) are the anticipated direction of a stress-reaction and baseline diurnal variation of parameters under study. If stress is expected to increase a parameter, measurements should be done when the parameter is naturally low and, vice versa, if stress is known to reduce a parameter, then measurement should be planned in the period when it is naturally high. If direction of stress-related changes is predictable, investigations can be performed on a twenty-four hour basis.