Much of the research on fescue toxicosis has concentrated on evaluating animal response to grazing endophyte-infected (E ) versus endophyte-free tall fescue or the effects of single toxins such as ergonovine (EN), ergovaline (EV), or ergotamine (ET) on animal performance. Such approaches have eliminated the opportunity to test the possible additive, synergistic, or antagonistic interactions of one or more ergot alkaloids with the other ergot alkaloids found in E tall fescue. This study was conducted to determine the effects of simultaneous exposure of pairs of EN, EV, and ET on the kidney adenosine triphosphatase (ATPase) system in vitro. Tests were performed using three separate rat kidney homogenates and were repeated four times at concentrations of 0, 75, and 200 μM. Individually, EN, EV, and ET induced dose-dependent inhibitions of kidney Na /K ATPase, with EN being most potent, followed by purified EV, and then by ET. The ergot alkaloids inhibited Mg2 ATPase to a lesser degree than Na /K ATPase, with EN again being the most potent toxin. Simultaneous exposure to any combination of the ergot alkaloid pairs tested (EV ET, EV EN, and ET EN) resulted in significant interactions (P < 0.05), indicating antagonistic effects on the inhibition of Na /K ATPase and Mg2 ATPase for most concentration combinations. These interactions suggest that in studies of the effects of any ergot alkaloid on animal performance, effects of other ergot alkaloids may also be present. Effects may not be additive, as was the case in this study, and the presence of one toxin may enhance or hinder the effectiveness of others.