Abstract δ-Aminolaevulinic acid (ALA) inhibited spontaneous contractions and contractions induced by acetylcholine, barium chloride or ouabain in rabbit duodenum preparations suspended in Ringe-Locke solution. Concentrations of ALA between 25–100 μg/ml reduced amplitude of spontaneous contractions and at levels abouve 200 μgALA/ml spontaneous contractions ceased. Muscle tone was decreased in proportion to logarithm of ALA concentration between 25 and 1000 μg/ml. Gut ALA leves were increased by the treatment and returned to control values after washing, and pharmacological effects were fully reversed. Neither tetrodotoxin nor atropine affected the gut's response to ALA. The response still occured in calcium-free, high-calcium, potassium-free and high-potassium bathing fluid. Effects of ouabian resembled those of ALA only when gut had been pretreated with atropine; pre-treatment with tetrodotoxin induced slight contracture. The observed pharmacological effects of ALA must be by actions additional to sodium pump inhibition.