Fig. 2. Upper tubules were initially stimulated with 25 nmol l^–1 5HT in saline and fluid was collected at 10 min intervals for a total of 30 min. The tubules were then maintained in 25 nmol l^–1 5HT (filled triangles) or received 0.1 µmol l^–1 of either RhoprDH₃₁ (filled circles) or leucokinin I (filled squares) at the solid arrow, and fluid was collected for a further 40 min at 10 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. RhoprDH₃₁ and leucokinin I had no effect on the rate of fluid secretion or the ion composition of the secreted fluid. Each point is the mean ± s.e.m. of 7–9 individual tubules. Addition of 1 µmol l^–1 5HT at the dashed arrow was used to confirm the viability of the Malpighian tubules.