Each column represents the mean ±s.e.m. of the change in MAP, measured from the baseline immediately preceding the hypoxic challenge, recorded over 5 min periods of hypoxia. The percentage O2 in the inspirate is indicated above the columns. Control animals (Group 1, ▪), after l-NAME, 10 mg kg−1, i.v. (Group 2, □), after l-NAME with SNP, 10 μg min−1 kg−1, i.a. (Group 3,) and after l-NAME with SNP, 20 μg min−1 kg−1, i.a. (Group 4,). *P < 0.05 and **P < 0.01, when compared with Group 1; †P < 0.05 and ††P < 0.01, Group 3 or 4 vs. Group 2.