The purpose of this study was to examine the effects of endophyte (Acremonium coenophialium) toxic tall fescue seed on serum prolactin, daily feed intake, and average daily weight gain in male rats. Thirty Sprague-Dawley male rats (218 g) were randomly assigned to treatments of (I) rat chow (50%) plus an endophyte-free tall fescue seed (50%), (II) same as I only restricted feed intake as III, (III) rat chow (50%) plus endophyte infected tall fescue seed (50%) and (IV) rat chow (50%) plus endophyte infected tall fescue seed (25%) plus endophyte free tall fescue seed (25%). Dry matter intake and average daily gain were lower (P less than .05) for rats fed treatment II and III than for those fed treatment I and IV, with treatment IV being lower (P less than .05) than treatment I. Body weights on days 4, 11 and 21 followed the same trend with the rats fed treatment I having higher weights at each weigh period than those on treatments II, III and IV; and those fed treatment IV having higher weights at each period than those on treatment II and III. Baseline concentrations of serum prolactin were similar among treatments. Serum prolactin concentrations were lower (P less than .05) in rats fed treatment III and IV than in those fed treatment II, and those fed treatment I were not different from all others on day 4. On day 11, rats fed treatment I, III and IV had lower (P less than .05) serum prolactin concentrations than those fed treatment II.(ABSTRACT TRUNCATED AT 250 WORDS)