A previously published working model for the regulation of chlorophyll formation has been tested studying early steps of chlorophyll and porphyrin biosynthesis in developing cotyledons of Helianthus annuus. The activities of delta-aminolevulinate synthetase (ALAS), delta-aminolevulinate dehydratase (ALAD), and the porphobilinogenase complex (PBGase) at any given time have been found to be strongly associated with endogenous developmental processes. Highest activities in darkness have been observed at times when maximum chlorophyll formation would have occurred had the plants been exposed to light. Only in the case of ALAS was the maximum activity in light much greater than that observed in the dark. Density labeling experiments and other data suggest that enzyme synthesis is mediated both by development and by illumination. Moreover, ALAS activity appears to be subject to inhibition, presumably by products of the porphyrin biosynthesis, as indicated by halflife experiments. Rapid enzyme degradation in the absence of light seems to be less probable. Slight ALAS activity in darkness is present as long as the plastids are not fully developed. In contrast to findings with cell cultures of tobacco, in Helianthus cotyledons ALAS certainly plays the main role in the regulation of chlorophyll biosynthesis. Nevertheless, increasing activities of the succeeding enzymes, located in the plastids, ensure that increased concentrations of delta-aminolevulinate (ALA) are drawn into the chlorophyll biosynthetic pathway. The experiments corroborate the suggestion that chlorophyll biosynthesis is controlled by different but interdependent mechanisms. The dominant regulatory mechanism is dependent on the stage of development.