Abstract When treated with exogenous 6-benzylaminopurine (BAP) cotyledons excised from 6-day-old etiolated sunflower seedlings ( Helianthus annuus L., cv. Tall single) exhibit stimulated chlorophyll (Chl) accumulation on subsequent illumination with white light. The influence of BAP on various enzymes of Chl biosynthesis was investigated. Cotyledons treated with BAP for 1.5 h before illumination show a greatly enhanced capacity for 5-aminolevulinate (ALA) production. The activities of ALA dehydrase and the uroporphyrinogen synthetase/uroporphyrinogen III cosynthetase complex in light are unaffected by a 24 h dark pretreatment with BAP. Under the conditions investigated, BAP-stimulated ALA production is reflected in a similar increase in Chl accumulation. Thus BAP stimulated Chl accumulation appears to be mediated through a rapid and selective enhancement of the capacity for ALA production.