The influence of carbon dioxide on the neuromuscular and haemodynamic effects of AH 8165, a new non-depolarizing muscle relaxant, was determined in nine healthy patients before the onset of minor elective surgery. Five patients (hypocapnic, group I) were maintained at an arterial Pco2 ranging from 25 to 31 mm Hg; four patients (hypercapnic, group II) at a Cco2 from 54 to 57 mm Hg. AH 8165 0.25 MG/KG (one-quarter of the dose required for intubation) was given by rapid central venous injection. Haemodynamic responses were similar in the two groups; there were increases in heart rate ranging from 45 to 60 per cent, increases in mean arterial pressure of 17 - 20 percent, and increases in cardiac output of 22-32 per cent. The intensity of neuromuscular blockade of the forearm muscles after AH 8165 was similar in the two groups, and there was no significant difference in recovery rates; group I patients were 80 percent recovered in 36.6 min. group II patients in 47.3 min. It was concluded that the activity of AH 8165 was not influenced by moderate changes in respiratory acid base status.