Four dairy cows were stressed by exposure to hot and cold environments in tests to determine the effect of environment on milk yield, somatic cell counts, and California mastitis test scores of milk from all mammary quarters and on bacterial counts of milk from infected quarters. Two cows were held in temperature-controlled rooms for successive 5-day periods at moderate (21 to 28 C), cold (-16 C), moderate, hot (36 to 37 C), and moderate environments. The cold and hot sequences were reversed for the other 2 cows. Temperature transmitters were surgically implanted in the skeletal muscles of the loin and gluteal regions; however, only one of these transmitters (gluteal region) functioned continuously throughout the experiment. At the end of this experiment, a transmitter was implanted in the gland cistern of a rear quarter of 1 cow, and the sequence of holding in the cold before the hot environment was used. Mean body temperature was approximately 1 degree higher (39.2 C) in the hot room (1 cow) and 3 to 4 degrees lower (35 C and 33 C), respectively, for 2 cows) in the cold room than that during the moderate temperature periods. A similar comparison showed that the mean intramammary temperature was 1 to 2 degrees higher (39.5 C) in the hot room and approximately 9 degrees lower (29.4 C) in the cold room. Exposure of the cows to hot and cold environments caused a greater loss in milk production in the 2 medium-yielding cows (23 to 28 kg/day) than in the 2 low-yielding cows (9 to 13 kg/day). The effect of the extreme temperatures on the somatic cell counts in uninfected quarters was limited to only a few quarters and was inconsistent (mean counts increased and decreased at both temperatures). The California mastitis test reactions showed no consistent changes during periods of heat and cold stress. Also, the effect of the environmental temperature on the intramammary infections also was inconsistent. The effect on bacterial counts appeared to vary with the type of organism. Some mean counts decreased in the heat and cold (Streptococcus agalactiae, Micrococcus sp), some increased (Pseudomonas sp), and another seemed independent (Streptococcus uberis) of the environmental temperature at which the cow was held.