PURPOSE: To assess retinochoroidal overexpression of heat shock proteins (HSP-70) induced by a transpupillary laser irradiation below the photocoagulation threshold. METHODS: Four pigmented rabbits were anesthetized and TTT was performed on the right eye using a 810nm diode laser (Iridis, Quantel-Medical, France) adapted on slit lamp (spot size: 1.3 mm, duration: 60 seconds; power 92-150 mW). Series of laser impacts were aimed at the posterior pole of the retina. Left eyes were used as control. Twenty-four hours after laser irradiation, a histological study was done on chorioretinal layers. Tissue samples were fixed in formalin and embedded in paraffin. A monoclonal antibody was used to detect HSP-70 immunoreactivity (mouse IgGl, SPA-810, Stress Gen, Canada), followed by a biotinylated goat antimouse antibody (Dako, Denmark), revealed by the avidin-biotin complex (Vectastain kit, Vector, USA) and the AEC chromogen. Retinal structures were further identified by HES coloration. RESULTS: The photocoagulation threshold was obtained for laser power at 150 mW. Under this threshold, HSP-70 immunostaining was the strongest for power 127 mW with a staining of some choroidal cells, including capillary endothelial cells. No HSP-70 immunoreactivity was observed on the retina. For the laser power 107 mW, HSP-70 reactivity was observed only in occasional choroidal cells. For the laser power 92 mW, as for nonirradiated eyes, no HSP-70 immunoreactivity was detected. CONCLUSIONS: Transpupillary 810 nm laser irradiation under the photocoagulation threshold induces choroidal HSP overexpression. This study concludes that choroidal HSP overexpression can be induced during TTT.