Three experiments were conducted with sheep to determine the effects of 1) increasing the dietary concentration of ergovaline and 2) environmental temperature on site of nutrient digestion and ruminal passage rates. Dietary ergovaline concentration, a measure of endophyte-alkaloid concentration, was modified by substituting ergovaline-infected for noninfected tall fescue seed. Ruminal and total tract OM, NDF and cellulose digestibilities were less (P less than .10) and ruminal fluid dilution rate (%/h) and fluid outflow (liters/h) were greater (P less than .05) when diets contained 3 ppm ergovaline than when diets contained 0 ppm ergovaline. An interaction (P less than .05) between dietary ergovaline concentration (0 vs 1.5 ppm) and environmental temperature (27 vs 34 degrees C) was found for OM intake and NDF and cellulose digestion. Ruminal fluid dilution and outflow rates decreased (P less than .01) even though water consumption increased (P less than .01) with increasing temperature. Rectal temperature increased (P less than .01) when ergovaline and ergotamine tartrate were fed and when sheep were exposed to increased environmental temperature. Under the dietary (and environmental) conditions described, our results suggest that alkaloid(s) present in infected tall fescue seed inhibit ruminal fiber digestion and increase the sensitivity of sheep to heat stress.