An extremely tall mutant (EMS 18-12), a bushy mutant (EMS 24-5), their parental variety Nirmal, and their double mutant recombinant were used to study individual and combined effects of genes producing opposite effects on plant height in periwinkle (Catharanthus roseus). Plant height of the extremely tall mutant (EMS 18-12) was controlled by epistatic inhibitory interaction between two independently inherited dominant genes, Et and H; Et producing extremely tall phenotype and H inhibiting Et. Both genes were inherited independently of plant height-reducing recessive gene by in the bushy mutant (EMS 24-5). Individually, genes Et and by increased and reduced plant height at harvest (when plant were 9 months old) by 90 and 25%, respectively, over parental variety. The double mutant recombinant (Etby) was taller than the bushy mutant (EMS 24-5) and variety, Nirmal but shorter than the extremely tall mutant (EMS 18-12) at different stages. At 1 to 7 weeks after germination, its height was 7.4 to 30.0% greater than the mid-parental value but 5.8 to 30.5% shorter than that expected on the basis of individual effects of genes Et or by. At the age of 4 to 9 months, its height was 5.4 to 40.1% greater than the mid-parental value and 5.6 to 44.1% (except at 5 months) greater than that expected on the basis of individual effects of genes Et or by, suggesting age-dependent epistatic interaction between the genes. No interaction effects were observed for leaf and root yields or contents of alkaloids in leaves and roots.