The effect of pH changes on the crystallization in solutions with an ion composition assumed to correspond to that of urine in the distal part of the distal tubule was examined by recording the number and volume of crystals with a Coulter Multisizer and by studying the crystal morphology with scanning electron microscopy at different degrees of volume reduction. The experiments were carried out with 100 ml samples at different starting pH values without and with 20% of dialysed urine (dU). The number of crystals increased in response to the volume reduction. In solutions without dU, 100 or more crystals with a diameter in the range 2.4-45 microm were observed already at a volume reduction of 40% when the initial pH was 7.28. For solutions with a pH of 5.80 and 6.45 the corresponding values were 60% and 80%, respectively. In the presence of dU, an appearance of crystals was recorded at volume reductions of less than 20%. In solutions with an initial pH of 5.80 and 6.45, the crystal number was greater with dU than without; such a difference was not recorded at pH 7.28. In samples containing dU, the mean crystal volume (MCV) varied very little when the sample volume was reduced. The same was found in solutions without dU when the initial pH was 5.80 and 7.28, whereas the MCV was greater in the samples with pH 6.45. Scanning electron microscopy of solutions reduced to 30-40% of the original volume showed that calcium phosphate had formed in solutions with a starting pH of 7.28 and 6.45. In solutions with pH 5.80 calcium oxalate crystals were observed with calcium phosphate.