The Cts mouse is a mutant of the Jcl:ICR strain with congenital cataract and small eyes in homozygotes. In the present study, attempts were made to measure postnatally the palpebral fissure and to examine the process of eyelid development by light microscopy in both normal (+/+) and homozygous (Cts/Cts) mice. The width of the palpebral fissure in homozygotes was significantly smaller than in unaffected mice. To investigate the process of eyelid development, tissue specimens from normal and Cts homozygous mice were prepared with hematoxylin and eosin, and observed by light microscopy on days 14, 16, and 18 of gestation and on days 0, 7, and 14 after birth. In both groups, the eyelid fused on day 16 of gestation and opened completely on day 14 after birth. The primordium of the orbicularis oculi muscle was defined on day 16 of gestation, and the primordia of hair follicles and Meibomian glands were detected on day 18 of gestation. In the present study, there were no significant differences in the process of eyelid development between normal and homozygous mice. Eyelid development has thus far been thought to be self- determining and not affected by tissue interaction, and the present findings support this line of thought.