Treatment of imbibed seeds with an atmosphere of 2.5 per cent. carbon dioxide is shown to be a powerful means of breaking the dormancy of seeds of a range of small-seeded legumes. On the bases of amount and rate of germination this method is at least as effective as, and usually superior to, holding imbibed seeds at 3-5°C for 3 days. Moreover, the carbon dioxide treatment was effective in most of the cases where cold treatment was ineffective. Some implications of these findings are indicated.