Diminished activity of uroporphyrinogen I-synthetase in the liver and other tissues may be regarded to be the primary genetic deficiency of acute intermittent porphyria (AIP). Increased production and renal excretion of delta-aminolevulinic acid (ALA) und porphobilinogen (PBG) are secondary phenomena. The neuropsychiatric symptomatology of AIP consists of neuropathy, vegetative crises and exogenous psychoses. In this study electromyographic and neurographic investigations were performed on 20 persons with AIP. 16 patients had experienced attacks of AIP, 10 of them including neuropathy. 4 persons showed the biochemical findings of AIP but had not yet had symptoms. In cases with persistent pareses following porphyric neuropathy denervation signs or sequelae were still present. In patients without clinical symptoms and in latent cases there were normal or borderline findings. Motor nerve conduction velocity was mostly decreased in combination with denervation signs and in a range that indicated a primarily axonal nerve lesion and consequent myelin damage rather than primary demyelinization. The mean motor conduction velocity of n. tibialis was somewhat lower in patients with porphyric crises without neuropathy than in latent cases without any clinical crises. The differences were not significant in other nerves. The findings are discussed under consideration of the electrodiagnostic results of other investigations and of neuropathological and clinical data.