Respiratory quotients for longleaf pine (Pinus palustris Mill.) seeds were generally lower than for slash pine (P. elliottii Engelm.). The values for longleaf pine peaked when the radicle emerged. Changes in lipid and soluble-sugar contents of longleaf pine seeds indicate that the rise in respiratory quotients reflected a change in respiratory substrate from lipids to sugars when the radicle emerged. Before germination started, protein contents of seeds of the two species were almost identical. During the first 10 days of germination and seedling development, protein levels in slash pine increased in all seed parts faster than those in longleaf pine. The differences in respiratory activity and nitrogen metabolism between the two species indicate that their metabolic pathways were differentially used during germination and early seedling development. Forest Sci. 16:350-355.