An infradian modulation with a 5-day period characterizes corneal mitoses in adult female inbred Lewis rats. The animals had been standardized, two per cage, with Purina Chow and deionized water freely available at 24 +/- 1 degree C and approximately 50% relative humidity, on six different sequences of light (L) alternating every 12 hr with darkness (D). The LD 12:12 regimen was staggered by approximately 4hr in the six environments to approximate, by sampling within 1 hr, sampling over a 24-hr LD span. Bedding was changed 3 days before each day of killing. During each of 8 consecutive days, one rat from each environment was anesthetized with ether and the eyeballs were removed. Mitotic indices, computed separately for each eye, were analyzed by linear and nonlinear least-squares analyses. For a prominent circadian component of mitotic activity, the 95% confidence interval (CI) extended from 23.84 to 24.85 hr, with a point estimate of 24.32 hr; for a second component of 117 hr, the Cl extended from 90 to 163 hr. Whether the approximate 5-day infradian rhythm is of ovarian or other origin is a problem for further study.