To understand the role of the seed coat in regulation of houndstongue seed dormancy, the effects of manipulation of seed coat integrity on seed germination and O2 uptake were studied. The results suggest that the seed coat of this weed regulates dormancy in part by interfering with the diffusion of O2 to the embryo. Scanning electron microscopy showed a network of ridges on the seed coat surface that were partially dissolved following 1.5 min of sulphuric acid scarification. Mechanical scarification removed fragments of the seed coat surface. Both scarification treatments stimulated seed germination. Supply of an elevated level of O2 also stimulated seed germination. O2 uptake by seeds imbibed in O2-saturated water was 150% higher than that for seeds imbibed in air-saturated water. Although all treatments that stimulated seed germination also stimulated O2 uptake, there was a lack of a consistent, quantitative relationship between increases in O2 uptake and seed germination in various experiments. This suggests that limitation of O2 availability to the embryo is not the only factor involved in regulation of houndstongue seed dormancy by the seed coat. Mechanical restriction of embryo expansion by the seed coat may also be important. Methanol-insoluble phenolics constituted < 1% of the total phenolic pool in the embryo. Their potential oxidation could not account for more than a small fraction of the previously reported massive stimulation of O2 uptake by the embryo upon decoating. The present O2 uptake and seed germination studies indicate that not all of the large increase in O2 uptake following decoating is essential for houndstongue seed germination.