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The cardiovascular effects of nitrous oxide were studied in thirteen dogs. Nitrous oxide depressed the myocardial contractile force in the deg as measured with the Brodie-Walton gauge. Cardiac output, central venous pressure, arterial blood pressure, and heart rate were not significantly changed. Sympathetic nervous responses were neither inhibited nor continuously stimulated by this agent. The effect of nitrous oxide on the efficiency of the heart is discussed. Nitrous oxide was introduced as an anaesthetic agent over one hundred years ago. Despite its wide use, there is a marked paucity of data regarding its effects on the cardiovascular system (Hamilton, 1963). This is especially true with regard to the effects of this agent on the intact, innervated cardiovascular system. This project was carried out to study the effects of nitrous oxide on myocardial function and haemodynamic parameters. Haemodynamic measurements included arterial blood pressure, central venous pressure and cardiac output. Measurements of myocardial function included maximum isometric systolic tension, rate of tension development with systole, rate of relaxation and heart rate. A Brodie-Walton strain gauge was used to measure isometric tension so that our data could be compared with those of others who have studied the effects of various anaesthetic agents using this instrument (