1. Four comparable groups of Shorthorn cows were used for an investigation of the effects of various amounts of hay, given with concentrates, on the fat content and yield of milk.

2. During the initial and final control periods all the cows received 16 lb. hay daily and about 4 lb. of a balanced concentrate mixture (flaked maize 50%, weatings 35% and decorticated groundnut cake 15%) per 10 lb. of milk produced. During the experimentaltreatment period of 7 weeks the groups received 12, 8 or 4 lb. hay, or 8 lb. coarsely ground hay, per cow daily respectively, with sufficient of a low protein concentrate to compensate for the reduction in hay, and the balanced mixture as in the control periods.

3. The change from 16 to 12 lb. hay did not affect the fat content of the milk, a small decline being attributable to the advance in stage of lactation, but the mean values for the last 2 weeks of treatment, adjusted for the differences in the initial control period, showed that the milk fat percentage for the groups receiving 8 and 4 lb. hay and 8 lb. ground hay had fallen by 1·16,1·12 and 1·72 respectively by comparison with that for the 12 lb. hay group.

4. A group of Friesian cows was included in the experiment, and received 4 lb. hay during the treatment period. The fall in the fat content of their milk was greater, but not significantly different from, that observed with the corresponding group of Shorthorn cows.

5. In the treatment period the milk yields tended to decline more rapidly than in the control periods, and this taken in conjunction with the falls in fat content was reflected in considerable falls in the yield of milk fat, ranging from 28·4 to 55·2% with diets containing less than 12 lb. hay. Both the fat percentage and fat yields recovered in the final control period.