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We used the freeze-fracture method to compare the intramembrane structure of the plasma membrane in inferior oblique (IO) muscles from six strabismus patients with normal IO muscles. Inferior oblique myectomy provided us with small pieces of IO muscle from three patients with superior oblique paresis, and from one patient each with V-pattern exotropia, V-pattern esotropia, and V-pattern intermittent exotropia. Samples of normal IO muscle were obtained from two men, aged 33 and 73 years, within 24 hours of their death. The excised muscles were separated at their belly part into orbital and global layers, and small bundles of muscle fiber were fixed and freeze-fractured. The size of intramembrane particles (IMPs) did not differ significantly between normal and strabismus-affected specimens (mean: 9.5 nm). The density of IMP in the 33-year-old man's sample ranged from 800 to 3300/microns2 with a mean of 1823/microns2, in the 73-year-old man's sample ranged from 100 to 2500/microns2 with a mean of 670/microns2, while in the affected tissues it ranged from 100 to 1000/microns2 with a mean 500/microns2. There were no significant differences in caveola size and density between normal and strabismus-affected IO muscles.