Abstract 1. 1. Taking into account available experimental data. a mechanism of action for aminolaevulinate dehydratase is proposed. 2. 2. Besides the formation of a Schiff base between the substrate and the enzyme, the existence of a minimal functional dimer necessary for activity is also considered, as well as certain characteristics of the type of interactions involved in the inter-subunit contact sites and the role of histidine residues in the transport of protons from and to the active center.