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Female rats treated with liothyronine or its solvent (control rats) were given an intravenous injection of [(14)C]-L-histidine. The amount of [(14)C]-histamine in various tissues was measured at 10 min, 60 min and 22 hr after the injection. In control rats the glandular stomach contained large amounts of [(14)C]-histamine at 10 and 60 min, with a sharp decline at 22 hr. The skin contained small amounts at 10 and 60 min, with a slight rise at 22 hr. In rats treated with liothyronine there was more [(14)C]-histamine in the stomach at 10 and 60 min, but not at 22 hr after the injection of [(14)C]-L-histidine. The results support the following conclusions: (1) The glandular stomach in the intact rat forms more histamine than do other tissues; the gastric histamine is in a rapid state of turnover and is likely to contribute substantially to the urinary output of histamine. (2) After treatment with liothyronine the rat has an increased histamine formation in the stomach; this is probably the main cause of the raised urinary histamine excretion in such a rat.