The cardiovascular effects of infusions of Althesin at various rates to supplement nitrous oxide anaesthesia have been studied in seven spontaneously breathing patients and11 patients ventilated artificially to normal PaCO2. During spontaneous breathing, increasing rates of Althesin infusion were associated with increases in heart rate and cardiac output. The modest decrease in arterial pressure (-5%) was the result of a decrease in vascular resistance. Increasing rates of Althesin rates of Althesin infusion (up to eight times the minimum infusion rate) caused dose-dependent decreases of arterial pressure and systemic vascular resistance, whereas heart rate and cardiac output were increased slightly at all rates of infusion.