After pilocarpine iontophoresis the change of sweat concentration during collection was studied by vapour pressure osmometry in 24 patients with cystic fibrosis and 24 healthy controls. There was a continuous but proportionate fall in sweat concentrations during the collection period. Mean (SD) initial sweat concentration in the control group was 154.4 (32.6) mmol/kg falling, after 50 microliters of sweat produced, to 92.9 (15.8) mmol/kg. In the cystic fibrosis group it was 315.9 (35.8) mmol/kg falling to 247.4 (24.9) mmol/kg. Despite different rates of fall in concentrations, separation of the two groups was maintained throughout. We conclude that there are implications for the potential improvement of the predictive value of the sweat test.