The C57BL/6J-mivit/mivit mouse has a retinal degeneration in which photoreceptor cells are lost slowly at a rate of about one row per month beginning at 8 weeks. ROS are severely disrupted by 4 months, but inner segments maintain a normal length through 6 months. In addition to photoreceptor changes, the retinal pigment epithelium is unevenly pigmented. The present study utilized histological and biochemical techniques to assess the effects of dark-rearing on the progression of the retinal degeneration in the mivit/mivit mouse at ages 4, 6, 8, 12, 16, 20, 24, and 28 weeks. Results of systematic morphometric evaluation indicated that the rate of loss of photoreceptor cells did not differ significantly from the rate determined for mivit/mivit animals reared in a standard light cycle. Furthermore, retinal detachment from RPE, the displacement of darkly-staining cells into the subretinal space and the influx of macrophage-like cells in the area of the ROS were still present in mivit/mivit animals reared in darkness. ROS of mivit/mivit seemed to be preserved for a slightly longer period of time in the dark-rearing condition. Rhodopsin levels in 4-week dark-reared mivit/mivit mice were 0.32 +/- 0.04 nmol per retina which was comparable to mivit/mivit mice reared under standard lighting conditions. At 20 and 28 weeks, rhodopsin levels decreased in mivit/mivit retinas to a similar level regardless of their lighting history. The findings of the study suggest that light deprivation does not retard the degeneration mivit/mivit retina. Results are discussed in comparison with effects of dark-rearing on other models of retinal degeneration.