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A study was made of the potential for manipulating urine volume and urea concentration in dairy cows. Four rations were tested. Rations high or low in rumen protein surplus (RPS) (1.0 vs. 0.1 kg RPS daily) were fed with or without NaCl 385 g daily added. Treatments were tested in four multiparous dairy cows according to a Latin square design. After an 11-day adaptation period, milk, faeces and urine production and composition were measured. Urine production was affected by RPS and NaCl. Differences in RPS were completely recovered as differences in urea excretion. Variation in urine volume resulted in differences in urea concentrations. Thus, a decrease in RPS leads to a small reduction in urea concentrations, unless a reduction in urine volume can be prevented.