The thermal dehydration test was performed in 12 patients with renal transplant and in 20 healthy subjects. The study was aimed at the evaluation of the effect of volume regulating hormones on electrolyte composition of thermal sweat in patients with renal transplant. Blood plasma renin activity (PRA) as well as plasma concentrations of aldosterone (ALD), vasopressin (AVP) and atrial natriuretic peptide (ANP) were determined before and after thermal dehydration in all the subjects studied. In all the subjects sweat was also collected after 15 and 45 minutes of exposition to heat and the concentrations of sodium, potassium and chloride were determined in all sweat samples. Significantly elevated PRA and ANP concentrations and significantly lowered plasma AVP concentrations but normal ALD levels were found before thermal dehydration test in all the patients with renal transplant. After the exposition to heat lasting 1 hour the direction of changes was similar, their magnitude was, however, different in renal transplant patients than in healthy subjects. In addition, lower concentrations of sodium and chloride in thermal sweat and lower total concentration of sweat solids were found in renal transplant patients than in healthy controls. No significant correlation was found between the plasma concentrations of the hormones determined and the electrolyte concentrations of thermal sweat both in the renal transplant patients and in healthy subjects. The results suggest that the volume regulating hormones have no effect on the electrolyte composition of thermal sweat induced by short exposition to heat both in renal transplant patients and in healthy subjects.