Two Latin squares balanced for carry on effects were used to study milk fat composition C12:0 to C18:2 of 12 cows fed high fat under environmental temperatures: cool 15 to 24C and a constant hot 32.2C with 60% relative humidity. Cows were fed daily: alfalfa-grass hay at 1.25% their body weight and one of three concentrates: control (without added lipids), oil (10% soybean oil), or fat (10% hydrogenated vegetable fat, iodine value 48) replacing corn by weight.The 32.2C temperature depressed proportion of milk fatty acids C12:0, C14:1 to 15:0, C18:1 and C18:2 while increasing C16:0 and C18:0. Both dietary oil and fat depressed concentrations of C12:0, C14:0, C14:1 to 15:O, C15:1, and C16:0 but increased C18:0 and C18:1. Compared with fat, dietary oil depressed C16:0 more while increasing C18:2. Thermal stress increased C18:0 more when the cows were receiving oil than fat. Cow differences were observed only with C18:l, and proportions of C18:0 increased and C18:1 decreased with repeated exposure to heat.