pl Materials and methods The research was carried out on 12 clinically healthy 4-year-old White Nobel goats. The animals were fed twice a day at 0700 and 1600 hours. The feed ration consisted of 0.7 kg of concentrated feedstuff (containing 16%-18% of protein), 0.4 kg of dry beet pulp, hay, and water (ad libitum). Ten days ahead of the research and during the research a strict light regime was kept of LD 12:12 (12 h of light from 0630 to 1830 hours and 12 h of darkness from 1830 to 0630 hours).

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... 30 to 0630 hours). Abstract: Research was conducted on 12 clinically healthy 4-year-old White Nobel goats. Renal activity examinations were carried out with the use of clearance methods where inulin was used as a testing substance to determine the level of glomerular filtration. The results allowed determination of inulin clearance, endogenic creatinine, sodium, potassium, and chloride levels. Filtered load, tubular resorption, and excretion in urine of sodium, potassium, and chlorides were marked. The research revealed diurnal variations of renal activity in goats. Most of the rhythms' acrophases occur in the phase of activeness between 1200 and 1600 hours. The resorption rhythm of potassium in the renal tubule differs from glomerular filtration by 12 h and its acrophases occur in the resting period. The main reasons for diurnal variations of the electrolyte excretion by kidneys are variations of the glomerular filtration rate. Feeding time does not affect the diurnal variations of renal activity. Due to the circadian glomerular filtration variations, resorption and secretion processes in nephrons, and, in consequence, the changes of the final urine composition, the kidneys lead to the organism's adaptation to changes in the environment and also in the circadian rhythm.