Histidine decarboxylase activity in hamster and rat brains were studied using a newly developed sensitive, direct radioenzymatic microassay. For our assay conditions, we determined a Km for L-histidine of 320 microM and a Vmax for histidine decarboxylase of 110 pmol histamine/hr/mg protein in rat hypothalamus. The regional distributions of both histidine decarboxylase and histamine levels were similar in the hamster and rat with the most activity in hypothalamus. Most of the histidine decarboxylase activity in rat hypothalamus was in the cytosol fraction. The developmental pattern of histidine decarboxylase in the fetal rat did not reveal a prenatal spike in activity. Histidine decarboxylase activity in rat brain reached adult levels by four weeks. Alpha-fluoromethylhistidine inhibited histidine decarboxylase activity almost totally in vitro at 10 microM and about 80% in vivo after six days of infusion (100 mg/kg/day) in all brain regions except the cerebellum. Likewise, histamine levels were depleted about 75% in all brain regions except the cerebellum.