Porphobilinogen deaminase (PBGD) is an enzyme involved in the synthesis of heme. Acute intermittent porphyria (AIP) is an inherited disease resulting from a reduced activity of PBGD. The symptoms seem to be due to a neurological dysfunction. Attacks of AIP are often provoked by conditions where the PBGD activity becomes insufficient as a result of an increased synthesis of heme in the liver. How this affects the nervous tissue is still unknown. It may well be that a reduced activity of PBGD in other tissues than the liver is of importance too. The aim of the present study was to examine the activity and the immunohistochemical localization of PBGD in the following tissues of wistar female rats: brain, heart, submandibular gland, liver, kidney, pancreas, ovary, stomach, duodenum, jejunum, ileum, colon and musculature. The PBGD activity varied considerably among the tissues. It was highest in the liver, 14 pkat/g, and lowest in the jejunum, 0.7 pkat/g. The immunohistochemical localization of PBGD was studied by antibodies raised against a 40 amino acid synthetic peptide that corresponds to a segment in the C-terminal part of PBGD. The study demonstrated that the PBGD immunoreactivity was not evenly distributed among the various cell types in a given tissue. Immunohistochemical reactions were pronounced in Kupffer cells in the liver, in smooth muscle cells of arteries and arterioles, in distal and collecting tubules in the kidney, in nerve axons in the brain and in ganglionic cells in the intestine. Especially, the immunohistochemical reaction in nerve cells is notable considering the nervous dysfunction in AIP.