In tropical rain forests, canopy openings created by treefall gaps are important sites for the establishment and growth of trees. Within such gaps new trees grow and a mature canopy is eventually attained. Therefore, gaps form a critical phase in the forest cycle because the species that are able to profit determine the floristic composition of the future stand. The author studied the factors that influence the establishment of seedlings and saplings of tree species and their growth in the Mabura Hill district in Guyana. The trees that are capable of growing in the deep shade of the understorey have potentially relative advantage with regard to canopy opening, over species that can only germinate and survive after gap creation. Insight in the process that, at least partly, determines presence and absence of tree species in the forest understorey, is valuable for the design of a silvicultural system. The study examined differences in survival and growth of tree seedlings in the understorey in relation to seed size. Also the growth and survival in canopy openings of tree seedlings differing in regeneration strategy (pioneer vs. climax) was assessed. The results are used to develop a model which aims at the evaluation of the relative importance of growth and seedling size for the regeneration of tree species in small and large gaps. Results show a positive relation between shade tolerance and seed size at very low light intensities. Furthermore, a strong correlation was found between seed size and seedling size. Larger seeded species tend to produce taller and heavier seedlings than small-seeded species. Results further showed that the relative growth rate of pioneer species, generally small seeded species, in deep shade is close to zero, whereas these species exhibited high growth rates in gaps. In contrast, the generally large-seeded climax species demonstrated a low, but positive, relative growth rate in the shade and only marginally higher growth in the more favourable light conditions of gaps.