Diathermy and cryosurgery have different effects on living tissue. Cell membranes are ruptured by cryosurgery, but the proteins are less affected than by diathermy. A short review of experimental cyclocryosurgery and the clinical results are presented. Data from the literature as well as evaluation of patients confirm that, as yet, the results are not satisfactory. The sequelae of freezing are very different in slow-and fast-freezing procedures. Therefore, the results in rabbits treated with a cryosurgical unit used for retinal detachment surgery (Amoils, -80 degrees C) were compared with the results using a cryoprobe cooled with liquid nitrogen (-180 degrees C). A permanent reduction in intraocular pressure was not achieved using cyclocryotherapy applied with the equipment used for detachment surgery (-80 degrees C). Repeated cryosurgery of this type did not result in a permanent pressure reduction. Histopathology demonstrated that the ciliary epithelium had regenerated and prolongation of the cyclocryotherapy applications did not result in a permanent intraocular pressure reduction. Lens opacities occurred in all rabbits of this series. Application of cryotherapy with a nitrogen-cooled cryoprobe (2.5 mm2 contact area) caused severe damage, resulting in phthisis bulbi. Another nitrogen-cooled probe (contact area of about 1 mm2 diameter) resulted in a permanent reduction in intraocular pressure, but the anterior parts of the lens became opaque. Histopathology revealed total necrosis of the ciliary epithelium and the basal membrane. Three months after cyclocryotherapy of this type the ciliary processes were atrophic and covered with connective tissue and regeneration of epithelial cells was not found in areas of such cryotherapy scars. Further experiments have been commenced including a reduction of application time, in order to find the optimum application technique resulting in permanent intraocular pressure reduction and the fewest possible side effects.