Spinal cord blood flow (SCBF) was measured in dogs before and following acute injury with 300 or 500 g-cm force (GCF). In addition, the responses to high and low PaCO2 and low PaO2 levels were studied. The hydrogen clearance technique was used and 0.3 mm platinum electrodes were placed in grey matter, central white matter or peripheral white matter of the L2 segment. The pre-trauma flows were: grey matter 12.5 +/- 2.7; central white matter 14.4 +/- 3.6 and peripheral white matter 15.1 +/- 4.2 ml/100g/min. Following a 300 GCF injury, a marked and progressive reduction in SCBF occurred in the grey and central white matter. This was present for the subsequent 4 hr of the study. The flow was lower than pre-trauma values during the second hour in the grey matter (9.0 +/- 1.4) and the third hour in the central white matter (10.8 +/- 1.8). By the fifth hour after trauma the flow in the grey matter was 5.0 +/- 3.5 and in the central white matter 9.7 +/- 1.5. In the peripheral white matter the SCBF was 10 +/-3.7 during the third hour but subsequently the flow increased to 11.5 +/- 3.9. Paired t-tests showed that this still significantly lower than pre-trauma levels. Two dogs showed a hyperaemic response which was persistent in one case but only temporary in the other dog. The vasodilatatory effect of CO2 was lost after trauma and in some cases a steal phenomenon was present. The sensitivity to an increase in CO2 was 0.48 +/- 0.23 ml/100g/min Hg before injury and this decreased to 0.0075 +/- 0.137 during the second hour after injury. The vasodilatation to hypoxia (30-40 mm Hg) was also absent but the vasoconstrictor effect to low PaCO2 appeared better preserved. These findings also applied to the peripheral white matter where the SCBF was not significantly reduced. The results were similar but more pronounced after 500 GCF injury. The results show that following injury the central areas of the cord become rapidly and progressively ischaemic. The peripheral white matter does retain a reasonably normal flow depending on the magnitude of the impact force. However, the vessels in all these areas lose their ability to respond to normal physiological stimuli.