The aim of this study was to establish variations in heartwood (percentage, diameter, radius, volume) in relation to tree height and sapwood thickness. The age at which heartwood formation begins and the maximum heartwood height in Tectona grandis trees were also investigated. Samples were taken from sixteen plantations in different zones of Costa Rica, ranging in age from 2 to 22 years. Our results showed that the sapwood is thicker at 2-10 years than at 10-22 years of age. Heartwood begins to form at the basal part of the trees at 2 and 3 years of age but does not appear at breast height until 3 to 4 years of age. The heartwood disappears at a height ranging from 0 to 90% of the total height, depending on the age of the tree. Finally, statistical predictive models were developed for sapwood thickness, heartwood radius, maximum heartwood height, heartwood percentage and heartwood volume, using the model Y = K0 + K1*(sapwood thickness) + K2*(relative sampling height) + K3*(diameter at breast height) + K4*(tree age). These models showed determination coefficients of 70%, 90%, 95%, 73% and 31%, respectively.