cAMP is a second messenger which plays a regulatory role in a wide variety of biological processes in organisms ranging from prokaryotes to higher eukaryotes, but knowledge of its role in macroalgae and vascular plants is limited. We modified cAMP levels in the macroalga Chara vulgaris thallus and studied the effects on thallus growth and gametangia development: db-cAMP (permeable analog of cAMP), adenylate cyclase (AC) activator, forskolin and theophylline (cAMP phosphodiesterase (PDE) inhibitor) were used to elevate cAMP levels, and the AC inhibitors 2'-dAdo and 2'-d3'-AMP were used to decrease them. The results suggest that in Chara vulgaris the cAMP pathway may regulate both vegetative thallus growth and gametangia development, and that these effects may depend on this second-messenger level. Elevated cAMP stimulated thallus growth and delayed gametangia development; decreased cAMP inhibited thallus growth and accelerated maturation of both antheridia and oogonia. These results suggest that the cAMP pathway participates in regulation of developmental processes in Chara vulgaris and that thallus growth and gametangia development require different cAMP levels in cells.