Well defined heat doses (temperature X time) were applied to normal rabbit corneas in an effort to determine thermal tolerance, and to examine the effects of heat on this tissue. A purely conductive heater was chosen to minimize intraocular penetration, and avoid findings attributable to nonthermal effects of inductive sources. The etched element heater was sewn to 38 rabbit corneas. Thirty-six were treated to temperatures of 38, 45, 52 and/or 59 degrees centigrade for durations of 5, 15, or 45 min. Three eyes were treated at each time-temperature interval and sacrificed at either time 0, 1 day or 1 week follow-up. Histologic examinations were performed on all corneas. A corneal haze was first noted at 45 degrees C X 45 minutes X 1 day follow-up. This correlated with a mild stromal edema on light microscopy. Higher thermal doses produced a spectrum of damage, with complete destruction of all keratocytes and endothelial cells at 59 degrees C X 45 min. At levels greater than 45 degrees C x 45 min, heat damage was noted to be increased at 24 hr followup. Some recovery was noted by 1 week follow-up, with the exception of the 59 degrees C X 15 or 45 min groups. These two heat doses induced a drop-out of cellular elements with evidence of disintegration and fragmentation of collagen fibrils. Conductive heating of up to 45 degrees C X 15 min appeared well tolerated by normal rabbit corneas.