The wall of the eye bulbs of 15 chick embryos, immediately after their transfer to Hanks fluid, was irradiated with a CO2 laser, focused on the 12th conjunctival papilla or on its prospective area. This 'in vitro' irradiation was performed under the following main parameters: (a) beam power congruent to 4W; (b) exposure time = 1/30 sec; (c) thickness of the fluid (Hanks solution) on the target surfaces = 0, 3 divided by 0, 7 mm. The same area of the eye bulb surface of other 15 chick embryos was irradiated 'in ovo' under the following conditions: (a) congruent to 4W; (b) = 1/10 sec; (c) (amniotic fluid + albumen) = 1 divided by 1, 5 mm. In the L/M analysis so far carried out, the lesions produced 'in vitro' and 'in ovo' showed a number of qualitative resemblances: i.e., disappearance of epithelial cells and numerous pyknoses in the cells left 'in situ'; decrease in thickness of the outer layer of the scleral mesenchyme, whose cells appeared closer to each other-owing to reduction of the intercellular spaces-, and often pyknotic. It is possible that the same mechanisms underlie such morphological aspects, observed in both groups of irradiated eyes. On the other hand, some quantitative differences were observed between the 'in vitro' and 'in ovo' lesions, the extent in surface and depth of the latter being appreciably lesser. Presumably the genesis of such differences could have been influenced by the physico-chemical differences between the fluids covering the target surfaces: Hanks fluid and, respectively, amniotic fluid + albumen.(ABSTRACT TRUNCATED AT 250 WORDS)