The absorption of onlay membranous and enchondral bone graft struts and paste was studied in immature New Zealand rabbits using a volume-displacement technique to determine the volume change in the grafts 8 weeks and 21 weeks after grafting. Membranous and enchondral bone paste does not survive as an onlay graft. Both enchondral and membranous bone graft struts undergo significant resorption (78% decreased volume in the enchondral grafts and 50% in the membranous bone grafts). These data support the clinical experience that both types of onlay bone grafts lose significant volume, and are in relative contradistinction to prior animal studies comparing membranous and enchondral onlay bone grafts.