The uptake and distribution in rats of the radiolabelled dipeptide L-alanyl-L-[U-14C]glutamine were investigated by whole-body autoradiography. Rats were killed 5, 30 and 180 min following bolus injection of the peptide via the tail vein and, as a comparison, 180 min after injection of [U-14C]glutamine; 30-microns sections of the animals were autoradiographed against an X-ray film. Tissue specimens from the remainder of the carcass were completely combusted and the 14CO2 formed was analyzed by liquid scintillation counting. Visceral organs especially exocrine glands and the mucosa of the alimentary tract showed an initially high labelling (5 min), whereas skeletal muscle and brain exhibited maximum radioactivity 30 min after peptide bolus. Muscle and intestine (19 and 10% of the dose injected, respectively) showed the highest 14C incorporation of all tissues investigated. The broad conformity in 14C distribution observed after peptide and glutamine bolus strongly indicates similar utilization of free and peptide-bound glutamine.