Figure 1. Ventilatory changes accompanying fourth ventricular infusions of morphine (1 micro gram/ml; n = 9); the nitric oxide (NO) donor, S-nitroso-acetylpenicillamine (SNAP; (10 sup -5 and 10 sup -4 M; n = 7); or the nitric oxide synthase (NOS) inhibitor, nitro-L-arginine (L-NA; 10 sup -6 and 10 sup -5 M; n = 8). The measured variables were PaCO2(left) and ventilatory drive (dp/dt60; right). The values are expressed as either change from baseline (Delta PaCO2) or % baseline (dp/dt60) and are presented as means +/- SE. *P < 0.05 versus baseline (see Table 2).