The scleral lamina cribrosa in the eyes of adult rats, hamsters, gerbils and guinea pigs was examined by ordinary histology and by scanning electron microscopy after soft tissue digestion. The complexity of the lamina, when mounted for scanning electron microscopy, was graded on a scale of 0 to 4.5 by three independent observers under × 60 magnification in a stereo microscope. The observers were unaware of the species and were offered the 44 specimens twice in random order. The average variance attributable to an observer was 22 ± 3% (SE) of the total variance of the gradings. The rat eyes had the least developed lamina cribrosa, with only 1–2 layers of sparse connective tissue. The mean complexity grading of 12 rat eyes was 1.6 ± 0.15. The lamina cribrosa of the eyes of gerbils and guinea pigs was much more developed with at least 3 layers of abundant connective tissue, the mean grades of complexity being 3.4 ± 0.09 and 3.5 ± 0.15, respectively, in 12 eyes of each species. The lamina cribrosa in the hamster eyes was somewhat more developed than that of the rat, but much less than that of the gerbil and guinea pig. The mean grade of complexity was 2.4 ± 0.14 in 8 eyes. In 6 pairs of rat eyes there was no correlation in grade of laminar complexity between the two eyes of the same animal. The present study makes the rat eye a candidate for experiments where a possible influence of the lamina cribrosa as such is un