Heat stress in dairy cattle results in decreased feed intake and decreased milk production. Past research has focused on describing the relationship between ambient conditions and changes in feed intake and milk production. This paper will address the important relationships that have been described in our research in the Brody Environmental Center. Key relationships will describe the effects of thermal balance measurements on decreased dry matter intake. Recently completed studies have allowed the measurement of changes in rumen fermentation and rumen function. The measurements have included intraruminal temperature changes associated with changes in management activities including feed intake, water intake, and cooling systems management.