The impact of dietary sodium on the incidence of nursing sickness in mink dams and on the average litter biomass of 28 and 42 day old kits was studied. One group (n = 115 including 12 barren females) was given a standard feed mixture with a natural content of 0.53 g NaCl/MJ and another group (n = 115 including 8 barren females) was given the same feed mixture supplemented with NaCl to a final content of 1.00 g/MJ. The average dam weight at weaning was significantly lower (P < 0.001) and the incidence of nursing sickness during the last part of the lactation period 3 times higher in the nonsupplemented group. The average litter biomass at weaning did not differ between the 2 experimental groups. A number of biochemical markers of preclinical nursing sickness, e.g. plasma aldosterone and osmolality, Na+ and Cl concentrations in plasma and urine, were studied during the last part of the lactation period and at weaning in 20 dams of the nonsupplemented group, in 10 dams of the salt supplemented group and, for comparison, in 5 + 5 barren females on the day corresponding to day 34 after parturition in nursing mink. The nonsupplemented group had significantly lower concentrations of sodium and chloride in plasma and urine and a significantly higher concentration of plasma aldosterone as compared to the salt supplemented group. Distinct signs of relative salt deficiency and preclinical nursing sickness thus characterized the nonsupplemented group throughout this period, while more blurred hints of electrolyte imbalances were noticed in the sodium chloride supplemented group at weaning. A beneficial effect of salt supplementation on the incidence of nursing sickness was shown; however, it remains unclear whether salt deficiency can cause nursing sickness or whether salt acts as an appetite stimulant preventing inanition and the development of the disorder.