Keywords: circadian rhythm-potassium and sodium urine excretion-shift work-night, day and afternoon shift succession Shift work requires biological rhythm readjustment in concordance with socially determined regimen of work and rest (1, 2, 4-6). According to certain authors the rhythm of potassium urine excretion belongs to the strictly regulated bioconstants that does not change in cases of disturbances of the natural rhythm of sleep and awakeness (3). There are only scanty data in the

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... re available about the circadian rhythm of potassium and sodium urine excretion in shift workers. There is, therefore, reason for investigating the diurnal dynamics of the excretion of these electrolytes in workers of successive three-shift regimen of work and rest.. Material and methods A total of 46 clinically healthy men aged between 28 and 40 years with length of service between 2 and 12 years were examined in our study. They were divided into 3 groups according to the regimen of work and rest and the intervals of exami­ nation. I st-12 shift workers from the chemical industry on successive shifts, (8 a. m.-4 p. m.; 4 p.m.-12 p. т.; 0 h-8 a. m.). Any workers were exami­ ned in 4-hours intervals (10 p. m.;-2 a. m.; 2-6 a. m.; 6-10 a. m.; 10 a. m.-2 p. m.; 2-6 p. m.; 6-10 p. m.) during work in day, afternoon and night shift as well as in rest days. II n d-12 operators in command halls of the thermo-electric power-station-Varna which shifts succeed each other every three days, and 12 operators of the thermo-electric power-station-Devnya which shifts succeed each other every four days. The examinations were performed in З-hours intervals during work in day, afternoon, and night shift. III rd-con­ trol persons of the administration-technical personnel with natural rhythm of sleep and awakeness. The examinations were done in 4-hours intervals during two successive days. Urine samples were collected after bladder emptying. In any sample potassium and sodium concentration was determined by using flame pho­ tometry. The results for the corresponding time interval were processed according to the method of variation and cosinor analysis. Results and discussion Potassium urine excretion has a characteristic diurnal dynamics when con­ trol persons are concerned. The minimal levels are found out during the interval between 10 p. m. and 2 a. m. (1,19=0,17 mequiv/h). Since 6 a. m. potassium con­ centration increases gradually and reaches its maximum in the interval between 10 a. m. and 2. p. m. (2,37=0,25 mequiv/h) that coincides with literature data