The role of the seed coat in inhibiting seed germination of Cynoglossum officinale, a rangeland weed in British Columbia, was studied. Pricking, acid and mechanical scarification increased germination significantly. These treatments increased O₂ uptake only slightly, with the exception of an initial burst in O₂ uptake by seeds mechanically scarified for 6 h. The results suggest that only a small percent of the six-fold increase in O₂ uptake following decoating, reported in an earlier study, is necessary for seed germination. Germination of seeds incubated at a high O₂ level was greater than that of seeds incubated in air, and O₂ uptake by seeds in O₂-saturated water was 150% higher than for seeds in air-saturated water. O₂ consumption by the seed coat was not a major factor in limiting O₂ availability to the embryo in the intact seed. These results taken together suggest that C. officinale seed coat regulates germination by restricting O₂ diffusion. The seed coat may also inhibit germination by mechanically restricting embryo expansion. The methanol-insoluble phenolics constitute <1% of the total phenolic pool in the embryo. Potential oxidation of methanol-insoluble phenolics can only account for a small percentage of the large increase in O₂ uptake following decoating. The relationship between seed phenolics and appearance of seed coat-imposed dormancy during seed development was studied. During the later maturation stages, the O₂ uptake by intact, and decoated seeds declined, with a larger decrease in intact seeds. In the most mature seeds there was a three-fold increase in O₂ uptake after decoating. Decoating also increased germination from 0% to 100%. As the seeds maturated, the methanol-soluble phenolic content of the embryo remained stable but that of the seed coat declined. This decline was associated with a large increase in thioglycolic acid-soluble phenolics, presumably lignin, in the seed coat. The results suggest that during later seed maturation stages, methanol-soluble phenolics in the seed coat are polymerized to lignin. The lignin may contribute to the formation of an O₂-impermeable and mechanically hard seed coat, which in turn may render the seed dormant.