In previous experiments systematic differences have been found in the morphology, carbon economy and chemical composition of seedlings of inherently fast- and slow-growing plant species. The present experiment was to investigate whether these differences persist when plants become larger. Plants of the inherently fast-growing Holcus Lanatus L. and the inherently slow-growing Deschampsia flexuosa (L.) Trin. were grown under standardized conditions and growth, photosynthesis, respiration and carbon and nitrogen content were followed over a period of 4 to 7 weeks. Differences in relative growth rate were mainly due to the higher leaf area ratio (leaf area; plant weight) of the fast-growing species. Rates of photosynthesis differed substantially when expressed on a leaf weight basis, but only slightly when expressed per unit leaf area. Although most parameters showed some ontogenetic drift, differences found for young seedlings persisted at least until plants reached a dry weight of circa 3 g. Therefore, at least for these two species, the conclusions based on interspecific variation relative growth rate of young seedlings apply to larger plants as well.