Seed respiration has been utilized as an index of seed vigour and has correlated well with seedling development. We examined the respiration process as affected by temperature and osmopriming during imbibition in two cultivars of sorghum [Sorghum bicolor (L.) Moench] differing in capacity to germinate at low temperatures. Respiration of imbibing seeds was measured with a specially built apparatus linked to an infrared gas analyser. Seed respiration at the beginning of the imbibition process strictly depended on temperature (T). T also greatly affected the total CO2 respired to reach the first germination event, which significantly increased as T declined from the optimum. Osmopriming affected seed respiration differently in the two cultivars. In the cold-tolerant cultivar (Brandes), osmopriming reduced the total thermal requirements and, at T close to the minimum for germination, the energy required to achieve the first radicle protrusion. In the cold-sensitive cultivar (H128), osmopriming lowered base T for respiration and reduced the requirements for total CO2 respired, allowing germination even at 10°C. The cold-tolerant cultivar always maintained higher respiration rates than the cold-sensitive one, showing a greater capability of mobilizing energy from reserves. Germination time (i.e. GR50) related well to cumulative seed respiration down to a threshold value of cumulative CO2, after which GR50 was more affected by T than was cumulative respiration. Seed water uptake significantly slowed with decreasing T, and was accelerated by osmopriming. However, water imbibition rate influenced the start of seed respiration, but not the subsequent rate of metabolic activity preceding germination. Seed respiration was a good indicator of the activation of germinative metabolic processes, although under extreme T conditions the seed populations still respired, even if at low levels, but did not germinate rapidly or completely.