Comparative physiology has much to contribute to our understanding of the circulation. For this purpose large animals until recently have been neglected as subjects of circulatory studies by modern technics, yet nature's means of solving the circulatory problems which these animals present are of great interest and, in case of domestic cattle (Bos taurus), of obvious practical importance. Intravascular and intracardiac pressures were recorded, and cardiac output and related functions were measured, or calculated, and related to body size. The effects of environmental stimuli and of phonation (“moo”) were observed. Interesting differences between the findings in the cow and those in normal man and in the giraffe are made the subject of comment.