The objective of this research was to investigate the involvement of ethylene and the enzyme endo-β-mannanase in lettuce seed germination under high temperature conditions. In an attempt to elucidate the relationship between these two factors, a study of the interaction between light and temperature on seed germination was taken as approach. Lettuce seeds of Everglades were incubated at 20.0; 27.5 and 35.0°C under constant light or dark. Germination of thermotolerant lettuce genotype decreased when seeds were imbibed under high temperature in the dark. Ethylene production correlated with enzyme activity (r2 = 0.85), and with germination (r2 = 0.90). Consequently, a high correlation (r2 = 0.96) was observed between enzyme activity in the micropylar region prior to radicle protrusion and seed germination.