Twenty eight grazing Holstein cows in mid lactation managed in a diurnal confinement system under artificial shade were utilized to evaluate the effects of cooling in the holding pen before milkings on milk production in Rafaela, Argentina (31º11’South) in summer 2002 (January 14th to March 8th). In the present paper, milk production responses are analyzed during heat waves, defined as periods when recovery hours per day, i.e. hours with THI below 72, are lower than 10. Two waves were recorded. During the first wave THI was above 72 for 15-17 hs and above 74 for 13-15 hs per day. ITH hs above 79 were recorded. Average milk production before (B), during (D) and after (A) were 24.4±0.3; 22.9±0.7 and 20.9±1.2 L/cow/day for the non cooled cows (P<0.001) and 25.6±0.4; 24.7±0.6 and 22.2±0.9 L/cow/day for the cooled animals (P<0.001). Production declines represented 14% for the non cooled and 10% for the cooled animals, thus reflecting a positive effect of cooling. No group returned to levels similar to those recorded before the heat wave. During the second wave, one month later, the daily hours above 72 and 74 ranged between 15-24 and 9-18, respectively. THI hs above 79 and 84 were recorded. Productions B, D and A were 22.5±0.4; 22.4±1.7 and 21.6±0.4 L/cow/day for the non cooled and 22.8±0.3; 22.5±0.5 and 22.1±0.3 L/cow/day for the cooled cows, respectively. During this second wave no significant differences were found for either group of cows, when comparing productions before and during the heat wave. There was a slight, though non significant, trend to a decrease in milk production immediately after the wave. A similar analysis was preformed on the basis of the well-being index (WI), an index developed for the Argentine Pampa region. WI calculations utilize not only temperature and humidity, but also temperature amplitude and cloudiness. Heat waves are longer when characterized with WI. Probably, this index overestimates effects of humidity, and should be revised for its utilization for characterizing dairy cows heat stress.