It is well known that the fatty acid composition of milk fat are altered due mainly to seasonal and dietary conditions1-12). Since seasonal changes are often accompanied by simultaneous changes in feed quality, it is necessary to control the environmental conditions in order to clarify the effect of specific environmental factors. The followings are a preliminary study to investigate the effect of high temperature on fatty acid in milk of dairy cows under the condition with diurnal changes of

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... mperature. Materials and Methods Two lactating Holstein cows weighing about 600kg, 6 months postpartum, were housed in a climatic laboratory of our institute. They were exposed to the control for two weeks, then exposed to the treatment for two weeks, and after such treatment they were returned to the control conditions for two weeks. Relative humidity was 60% throughout the experiment. The cows were given feeds according to the Japanese Feeding Standard (1974). Rations, offered each day, consisted of 3.8kg rice straw wafer, 3.2kg alfalfa hay cube, 2.8kg beet pulp and 10.5kg (for cow A) or 12.2kg (for cow B) concentrates. Milk yield and feed consumption of cows were measured daily. Milk fat contents were measured for the last seven days of each period as described by OSHIMA and FUSE13). Milk samples used for fatty acid analysis were frozen soon after milking of the last three days of each period. Milk fat was extracted with chloroform-methanol by the method of FOLCH et al.14). Portions of chloroform layer were dried up and methylated with HCl-methanol under a positive nitrogen atmosphere15). The methylat-