Dairy operations located in many temperate climates experience a decline in milk production associated with short-term moderate heat stress during the summer months. The duration and intensity of this heat stress differs dramatically from the sustained, intense heat experienced in many tropical and subtropical countries where the vast majority of heat stress research has been conducted. As such, many of the strategies utilized to ameliorate production losses associated with sustained, intense heat may not be effective in temperate climates. The objective of this research was to characterize the production responses of lactating dairy cows during and after short-term, moderate heat exposure and, to determine if feeding a fungal culture, Aspergillus oryzae, during a 5-day heat stress period, could effectively alleviate the associated production losses. In a two-period, cross-over design, eight mature lactating cows were given a total mixed ration with or without Aspergillus oryzae. Each 15-day period consisted of a 5-day thermoneutral phase, a 5-day heat stress phase and a 5-day thermoneutral recovery phase. When exposed to moderate heat stress for a 5-day period, cows experienced a rise in vaginal temperature and a decline in dry-matter intake. Following the 5-day heat stress phase, milk yield declined by a factor of 0·09. Supplementation with Aspergillus oryzae had no effect on vaginal temperature, dry-matter intake, water intake, milk yield or milk components. These data indicate that short-term, moderate heat stress, which occurs during the spring and summer months in temperate climates, will significantly decrease production in the lactating cow. Addition of fungal cultures to the diet during the period of imposed heat did not ameliorate production losses associated with this type of heat stress.