Prophylaxis of heterotopic bone formation is reliably accomplished via postoperative irradiation. Because of apparent need to initiate irradiation within l-5 days post surgery, this treatment is however quite painful. An analog study was therefore designed to assess comparative efficacy of pre versus post op irradiation.. Decalcified bone matrix pellets (10 mg each) were bilaterally implanted superficial to the roughened anterior thigh fascia of 95, 30 day old, Long Evans rats. irradiation (2 days pre-implant, Rats were randomized to, a) time of 1 hour pre-implant or 2 days post-implant), b) dose (800, 1800, 2400 or 3000 cGy in 1 fraction), and, plus 2 cm margin. c) target volume (left or right implant site Implants were removed at postop day + 16 and subject to morphometric and microradiographic analysis of percent bone development within each extracted pellet site. Results indicate that postop rats formed no bone at any of the dose levels. One hour and 2 day preop rats formed 7.6% and 10.0% bone respectively at 800 cGy. With the exception of one 2400 cGy rat in each preop group, formed no bone at 1800, all other 1 hour and 2 day preop rats 2400, and 3000 cGy dose levels. Analysis of variance showed no significant differences among any of the 3 treatment groups at any dose level. To further control for unknown host variables affecting the degree of bone formation, bone formation within each rat's irradiated implant was subtracted from degree of bone formation of the unirrddiated pellet site to thus form percent bone formation scores. Difference scores among postop, 1 hour preop and 2 day preop groups respectively were similar at 800 cGy (30.2, 20.8, 37.5%), 1800 cGy (35.7, 46.6, 37.3%), and 3000 cGy (42.1, 38.7, 35.7%). At no dose level did the discrepancy between difference scores reach statistical significance. Results of microradiographic analysis of percent bone formation, and densiometric analysis of comparative degree of calcium incorporation were similar to morphometric findings. Results suggest no differences at dose levels employed in terms of suppression of bone formation among rats irradiated postoperatively versus 1 hour or 2 days preoperatively. This suggests efficacy of irradiation delivered shortly prior to stimulus for proliferation to subsequently inhibit such proliferation. We are currently comparing utility of pre vs post op irradiation at lower doses (to 300 cGy) and at longer time intervals to sacrifice (48 days). Current results hint that development of heterotopic bone following hip surgery among our patients may be preventable by irradiating relevant hips shortly prjo_r_ to surgery at currently employed post operative dose levels and thus spare such patients the discomfort associated with postoperative irradiation.