The importance of light to development of Pinus resinosa seedlings for a short time following seed germination was studied. Treatment of seedlings with photosynthesis-inhibiting herbicides (monuron and atrazine) partially inhibited cotyledon expansion. Expansion of primary needles was light-dependent since darkness stopped it. When young seedlings were exposed to 14CO2 in the light, some of the label accumulated in root tips and primary needles, and some was retained in cotyledons and hypocotyls for at least 3 weeks. Although measurable photosynthesis occurred in both cotyledons and green hypocotyl tissues, the rates were much higher in cotyledons. In intact seedlings, expansion of primary needles was not significantly influenced by exposure of hypocotyls to light but in those from which cotyledons had been removed, exposure of hypocotyls to light did affect their expansion. Thus green hypocotyl tissues appeared to be important for expansion of primary needles only in the absence of photosynthesis in the cotyledons. Some growth of cotyledons, radicles and hypocotyls also depended on current photosynthate in addition to megagametophyte reserves.