It is a well-established fact that during the summer months, particularly July and August, the chemical quality of milk tends to be poor. The samephenomenon has been reported in Germany (i) and in the United States of America(2, 3). The individual factor or factors responsible have not been thoroughly established, although the work of Ragsdale and Turner (4) andHays (5) shows clearly that the fat content of milk tends to be reduced as the environmental temperature increases. The extent of the summer depression in milk quality in this country is much greater during hot dry summers. Cranfield(6) and Golding et al. (7) found exceptionally poor-quality milk during the summers of 1929 and 1933 respectively. In both these years most cows were subjected to a higher temperature than usual and also to a variety of other unusual factors such as the changes in nutrition occasioned by dried-up pastures. The work reported here was planned in order to throw some light on the influence of temperature, and whether it may be considered solely responsible for low-quality summer milk or whether other factors such as nutritional ones may be partially responsible.