Fig. 3. (A) H₂S dose–response curves for hagfish dorsal aortas (filled circles; N=22) and efferent branchial arteries (filled triangles; N=8) bubbled with room air, and dorsal aortas bubbled with 100% nitrogen (open circles; N=10). Aortas bubbled with nitrogen were significantly (^*) more sensitive to low concentrations of H₂S than air-bubbled aortas and appear to have a two-phase response to H₂S. H₂S at 100 and 300 µmol l^–1 produces a significantly greater response in normoxic efferent branchial arteries than normoxic dorsal aortas (). (B) H₂S dose–response curves for lamprey dorsal aortas bubbled with room air (filled circles; N=8) or during moderate hypoxia (open circles; N=8). Hypoxic vessels were significantly (^*) more sensitive to H₂S. Values are means ± s.e.m.