When grown in darkness the photomorphogenic lip1 mutant of pea (Pisum sativum L.) has a slender stem, expanded leaves, prolamellar body (PLB) lacking plastids with the size of chloroplasts and a low level of phytochrome A. The lack of PLBs in a dark-grown material (lip1) created a possibility to further study the regulation of their formation in relation to plant development. Inclusion of a cytokinin, 2-isopentenyladenine (2iP), in a medium supporting growth of the pea seedlings in darkness was found to reduce epicotyl length in the wild type. In lip1 the formation of a slender stem was inhibited and a short epicotyl developed. Furthermore, leaf expansion was inhibited, the plastid size reduced and the formation of PLBs induced. The PLB formation in lip1 was not accompanied by an increase in the amount of protochlorophyllide (Pchlide) or Pchilde oxidoreductase (POR). In the presence of 2iP the level of phytochrome A protein was increased in lip1 and the POR mRNA levels decreased in both lip1 and wild-type plants. The chloroplast characteristic trans-3-hexadecenoate acyl group of phosphatidylglycerol, present in the plastids of dark-grown lip1, was not influenced by 2iP. Thus, not all photomorphogenic processes reacted similarly in the lip1 mutant, but leaf expansion and plastid differentiation, including PLB formation, seemed to be regulated by the same signal transduction chain. Exogenously applied brassinolide could rescue neither dark- nor light-grown defects of the lip1 mutant. Thus, cytokinins but not brassinolides seem to be involved in the regulation of certain characteristic traits of skotomorphogenesis in pea, including plastid development and PLB formation.