Fig. 2. Time course of the effects of glutamine and cysteine on secretion rates of Drosophila tubules. Values are means ± S.E.M. Filled circles represent control tubules in amino-acid-free Drosophila saline (N=7), filled squares represent tubules exposed to 20 mmol l^-1 cysteine at t=40 min (N=12), and open circles represent tubules exposed to cysteine at t=40 min, and then to both 20 mmol l^-1 cysteine and 20 mmol l^-1 glutamine at t=80 min (N=3).