Levels of histamine and its major metabolites in brain, tele-methylhistamine (t-MH) and tele-methylimidazoleacetic acid (t-MIAA), were measured in rat brains up to 12 h after intraperitoneal administration of L-histidine (His), the precursor of histamine. Compared with saline-treated controls, mean levels of histamine were elevated at 1 h (+102%) after a 500 mg/kg dose; levels of t-MH did not increase. Following a 1,000 mg/kg dose, mean histamine levels were increased for up to 7 h, peaked at 3 h, and returned to control levels within 12 h. In contrast, levels of t-MH showed a small increase only after 3 h; levels of t-MIAA remained unchanged after either dose. Failure of most newly formed histamine to undergo methylation, its major route of metabolism in brain, suggested that histamine was metabolized by another mechanism possibly following nonspecific decarboxylation. To test this hypothesis, other rats were injected with alpha-fluoromethylhistidine (alpha-FMHis; 75 mg/kg, i.p.), an irreversible inhibitor of specific histidine decarboxylase. Six hours after rats received alpha-FMHis, the mean brain histamine level was reduced 30% compared with saline-treated controls. Rats given His (1,000 mg/kg) 3 h after alpha-FMHis (75 mg/kg) and examined 3 h later had a higher (+112%) mean level of histamine than rats given alpha-FMHis, followed by saline. Levels of t-MH and t-MIAA did not increase. These results imply that high doses of His distort the simple precursor-product relationship between histamine and its methylated metabolites in brain. The possibility that some His may undergo nonspecific decarboxylation in brain after His loading is discussed. These findings, and other actions of His independent of histamine, raise questions about the validity of using His loading as a specific probe of brain histaminergic function.