Abstract: The modular construction system includes a plurality of lightweight, load-bearing composite panels to be used in the walls, ceilings and/or roofs of buildings, as well as for partitions within a given building. Each panel includes sheet-like skins with a foamed plastic core therebetween. At each end of the panel is a structural unit that is fabricated from sheet stock, the structural unit including a transverse section having a longitudinal groove formed therein. When two panels are abutted in an end-to-end relationship, the grooves form a conduit for electrical wiring, communication lines and plumbing. Each structural unit also has a thermal barrier so that the structural unit is resistant to the transmission of heat therethrough. The panels are connected in their end-to-end relationship by means of interfitting lugs and slots on the structural units. Additional structural units extend across the top and bottom of each panel.