Nine yearling crossbred beef heifers, Bos taurus L., were used to examine physiological responses to horn fly, Haematobia irritans (L.), infestation. Heifers were stanchioned indoors in individual environmentally controlled rooms. On day 0, each animal received 0, 500, or 1,000 horn flies. Fly numbers were adjusted daily to maintain an appropriate infestation for each heifer. Feed intake, respiration rate, and rectal temperature was recorded daily. In addition, blood samples were collected from each animal on days 0, 12, and 33 for serum constituent analysis. To monitor metabolic hormone status, intensive blood samples were also collected hourly for 6 h on days 0, 12, and 33. Throughout the period of treatment, feed intake values were similar among treatments resulting in comparable body weight at the end of the trial. Respiration rates on each day were similar among groups. Rectal temperature was also unaffected by horn fly infestations. Serum electrolyte analysis revealed that neither Na, K, Cl, Ca, P, nor Fe differed among treatments. Likewise, HCO3/CO2 ratio, anion gap, and serum osmolality did not differ among treatment groups. Major indicators of nutrient status (glucose, cholesterol, triglycerides, urea N, creatinine, uric acid, albumin, globulin) and insulin, growth hormone, and prolactin were also unaffected. Serum bilirubin and enzyme (alkaline phosphatase, creatine phosphokinase, gamma-glutamyl transpeptidase, lactic dehydrogenase, aspartate amino transferase, alanine amino transferase) concentrations were similar in control heifers and those infested with horn flies.(ABSTRACT TRUNCATED AT 250 WORDS)