To investigate the effect of rod activity on color perception with light adaptation, chromaticity shifts of monochromatic test lights were measured as a function of background field intensity at 17 deg in the nasal field of view. The measurements were performed both after complete dark adaptation and during the cone-plateau period at a mesopic test intensity level of 15 photopic trolands. To clarify the mechanisms underlying the chromaticity shifts obtained, six supplementary experiments were performed. The results of the experiments strongly suggest that at scotopic background intensities, light adaptation of rods, both within and adjacent to the test area, may reduce rod signals triggered by the test light and thereby produce marked chromaticity shifts with light adaptation. At mesopic background intensities, cones in the background field become activated and may influence the chromaticity shift with light adaptation both by suppressing signals from rods elicited by the test light and by producing a selective chromatic adaptation.