In the course of plastid development there are changes in the permeability of the envelope membranes. An investigation of the kinetics of transport with largely uncontaminated and intact etioplast/etiochloroplast preparations from greening Avena sativa laminae demonstrates: (a) that etioplasts already possess specific translocators for the transporation of orthophosphate, dihydroxyacetone phosphate, 3-phosphoglycerate ("phosphate translocator"), and dicarboxylic acids ("dicarboxylate translocator"); (b) that changes in the rates of uptake during development are mainly due to changes in velocity for specific transport and not due to changes in the affinity for transport (K(m)) or nonspecific permeation. The very low competitive inhibition of transport of orthophosphate by dihydroxyacetone phosphate and 3-phosphoglycerate, observed for developmental stages corresponding to up to 3 hours of illumination of etiolated tissue, is discussed with respect to the possibility of an early phosphate transport mechanism that is different from the phosphate translocator of more developed plastids.