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Abstract Sixteen wether lambs (37 ± 3.7 kg) cannulated at the rumen and cecum were used in a 2 × 2 factorial experiment. Lambs were adapted for 15 d to a basal forage diet (50% prairie hay:50% oat straw; 5.8% CP) or basal plus 189 g · head−1 · d−1 of cottonseed meal (CSM; 42.5% CP). Starting on d 16, lambs in each diet group were infused via cecal cannulas with 2 g neomycin sulfate or .15 M NaCl. On d 24, ruminal, cecal and blood samples were taken at −2, 0, 1, 3, 6, 9, 12, 16, 20 and 24 h after CSM supplementation. Supplementation increased (P < .10) serum insulin and decreased (P < .10) somatotropin, but it had no effect on serum free fatty acids. Cecal antibiotic infusion decreased serum free fatty acids (P < .05). Protein supplementation increased (P < .10) serum urea-N, and antibiotic infusion increased (P < .10) serum urea-N in supplemented, but not in unsupplemented, lambs. Supplementation increased (P < .10) cecal NH3 concentrations at 0, 6, 9, 12, 16 and 20 h, whereas antibiotic had no consistent effects. Antibiotic increased (P < .10) ruminal NH3 at all times, except 12 and 20 h, in supplemented, but not in unsupplemented, lambs. Supplemented, antibioticinfused, but not supplemented, saline-infused, lambs had lower (P < .10) ruminal acetate (mol/100 mol) than either unsupplemented, saline or antibiotic-infused lambs at 3, 9 and 12 h. Supplementation with CSM increased cecal total VFA (P < .10) at all sampling times. Cecal acetate was higher and propionate lower (P < .10) at most sampling times in unsupplemented, antibiotic-infused lambs compared with lambs in other treatment groups. Cecal antibiotic infusion appeared to enhance N recycling in CSM-supplemented lambs but not in unsupplemented lambs. Copyright © 1988. American Society of Animal Science . Copyright 1988 by American Society of Animal Science.