Objective—To assess the acetaminophen absorption test (APAT) for use in determining function of the reticular groove reflex in lambs. Animals—12 Baluchi lambs. Procedures—2 consecutive APATs were performed at each of 3 developmental stages (stage 1, before weaning; stage 2, at weaning; and stage 3, after weaning). Lambs suckled a test solution consisting of acetaminophen and barium sulfate and 1 week later were tube fed the same test solution. Abdominal radiographs were obtained immediately after administration of the test solution. Plasma acetaminophen concentrations were determined before and 30, 60, 90, 120, 150, and 180 minutes after intake of the test solution. Results—Closure of the reticular groove after suckling the test solution was confirmed in all 12 lambs at stage 1, in 8 lambs at stage 2, and in 0 lambs at stage 3. Maximum plasma acetaminophen concentrations and area under the plasma acetaminophen concentration-time curves from 0 to 180 minutes were significantly higher in lambs suckling the test solution, compared with values for tube-fed lambs. Receiver operating characteristics analysis revealed that the plasma acetaminophen concentration at 60 minutes after administration was best suited to determine closure of the reticular groove in lambs. Conclusions and Clinical Relevance—Results suggested that the APAT can be a useful diagnostic instrument to assess function of the reticular groove reflex in lambs. We propose a cutoff value for the plasma acetaminophen concentration of 25 g/mL at 60 minutes after administration to determine function of the reticular groove mechanism in lambs. (Am J Vet Res 2009;70:820–825)

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