In dairy cows high ambient temperatures are associated with a decline of milk yield. The extent of milk yield decline observed in heat stressed cows depends on a series of factors. High yielding cows are more sensitive to heat and also stage of lactation affects response of dairy cows to high ambient temperatures. We compare milk production in dairy Holstein cows spending hot summer months in their peak production (about 50 days of lactation), about 100 days, 200 days and 300 days of lactation. The lowest milk yield in 305-d lactation was found in cows spent summer months in their 300-d of lactation, the highest milk production was in cows spending hottest period of the year in their 100 days of lactation (and so calved in spring months).