Fig. 1. Effects of extracellular pH (pH_o) on tension of unstimulated efferent branchial arteries (EBA; N=11) and anterior cardinal veins (ACV; N=11) in Hepes buffer, and on vascular resistance of rainbow trout gills (11 gills from four trout) perfused with phosphate-buffered saline. Values are means ± s.e.m. EBA and ACV values are significantly different from their respective control (pH 7.8) except at pH 7.4; all vascular resistances except at pH 7.4 and 7.0 are significantly different from that at pH 7.8.