Fig. 1. Upper tubules were initially stimulated with 25 nmol l^–1 5-hydroxytryptamine (5HT) in saline and fluid was collected at 10 min intervals for a total of 20 min. The tubules then received 1 µmol l^–1 of either 5HT (filled squares) or ZooneDH (filled triangles) and fluid was collected for a further 55 min at 5 min intervals. The rate of fluid secretion (A) was calculated after measuring the diameter of each droplet, and ion-selective microelectrodes were used to measure the K⁺ (B), Na⁺ (C) and Cl^– (D) concentration of each droplet of secreted fluid. ZooneDH and 5HT stimulate fluid secretion to a similar extent and both increase the Na⁺ concentration of the secreted fluid at the expense of K⁺ over time. Each point is the mean ± s.e.m. of 6–9 individual tubules.