Cattle ingesting endophyte-infected tall fescue (EIF) under heat stress often suffer from fescue toxicosis that can be manifested in decreased thermoregulatory ability. An experiment was carried out to examine the effects of nitroglycerin (NTG) on thermal status of yearling beef steers ingesting EIF under diurnal heat challenge in a 2x2 factorial arrangement of treatments. Animals were first maintained under thermoneutral zone (TNZ; constant 19¢XC) for 4 days then for 14 days of cyclical heat stress (26-36¢XC; figure 1). In Phase 1, animals were fed EIF diet at 22 ƒÝg ergovaline.kg-1.day-1 while in Phase 2 this level was doubled during the last 10 days of heat challenge. Core body temperatures (intraperitoneal; IP, intragastric; IG, and rectal; Trec) significantly increased (P < .05) to reach a maximum daily average level 4 days after onset of heat cycle, while respiratory rates (RR) and all skin (shoulder, rump and tail) temperatures (Tshoulder, Trump and Ttail, respectively) increased instantly (P < .01) during the gradual switch to heat (day 5) in both phases. In Phase 1, EIF did not induce hyperthermia but caused an elevation in values of Tcore change from TNZ baseline during peak heat hour, while there was no differences in average daily values of RR (P > .55) or skin temperatures (P > .12) among treatment groups. However, NTG enhanced peripheral circulation by decreasing the differences between Trec and skin surface temperatures. In Phase 2, EIF caused a significant elevation (P < .05) in all Tcore average daily values starting on day 14, and was maintained until the end of trial on day 20, while NTG abrogated the hyperthermic effects when administered along with EIF. Phase 2 was similar to Phase 1 in that there were no alterations (P > .05) in daily averages of RR or skin temperature between treatment groups. Nitroglycerin administration significantly (P < .05) decreased the temperature difference between rectal and skin surfaces, indicative of enhanced peripheral circulation.