Two experiments were conducted to determine whether administering a dopamine antagonist to steers fed endophyte-infected (E+) tall fescue would increase serum prolactin (PRL) and reduce rectal temperature. Steers in both experiments were housed in environmentally controlled chambers (32 degrees C; 50% relative humidity). In Exp. 1, 10 steers were allotted randomly to receive s.c. injections of either 0, .006, .03, or .06 mg of Ro 24-0409 (dopamine antagonist)/kg BW. The experiment was designed in four phases: endophyte-free seed (E-) without antagonist (d -11 to 0); E- with antagonist (d 0 to 7); E+ with antagonist (d 7 to 28); E+ without antagonist (d 28 to 38). In Exp. 2, 22 Holstein steers were allotted randomly to the same treatments and design, except three steers were maintained on E- without antagonist. Steers were fed individually with intakes measured daily. In Exp. 1, feed intake and rectal temperature were not improved (P > .05) by antagonist injection. In Exp. 2, antagonist injections increased (P < .05) PRL. Ingestion of E+ decreased (P < .05) feed intake and serum PRL. Antagonist injection decreased (P < .05) rectal temperature and increased (P < .05) serum PRL.