Fig. 4. Effects of 10^–7 mol l^–1 endothelin (ET-1) on the normalized diameter of the main artery to the head (normalized to the diameter under control conditions) in Xenopus laevis tadpoles. One group of tadpoles was preincubated for 20 min (black bar) with L-NAME (10^–4 mol l^–1) in Ringer’s solution prior to injection of endothelin. The other group was preincubated with Ringer’s solution alone for 20 min (black bar) prior to injection of endothelin. The arrowhead indicates the time of endothelin injection. Values are means ± S.E.M. (N=6); symbols and asterisks indicate significant differences from the control period.