The effect of haemorrhagic hypotension on the incidence, frequency and relative amplitude of vasomotion in rat diaphragm microcirculation was assessed by laser Doppler flowmetry (LDF). Graded bleeding to four hypotension levels (80, 60, 40 and 30% of the control state) were performed in 24 Sprague-Dawley rats. The incidence of vasomotion was 83% in the control state, 96% at the 80% level, 100% at the 60% level, 96% at the 40% level, and 46% at the 30% level. The median fundamental frequency of vasomotion determined manually during the control state and at the hypotension levels (in descending order) was 4.11 (range, 3.29-5.58) cycles min-1 (cpm), 4.48 (3.21-5.92) cpm, 4.20 (3.5-5.56) cpm, 4.01 (3.33-5.36) cpm, 3.71 (3.25-4.49) cpm (P < 0.01 from the fundamental frequency at 80 and 60% hypotension levels). The median relative amplitudes determined manually during the control state and descending hypotension levels were 44.5% (range, 24.9-135.9%), 69.4% (26.6-147.2%), 84.0% (40.3-177.1%) (P < 0.01 from resting and last stage of bleeding), 90.40% (26.2-189.6%) (P < 0.01 from resting and last stage of bleeding), 69.2% (35.6-93.2%). We concluded first that during the resting condition, vasomotion was frequently present in diaphragm microcirculation, which is distinct from other vascular beds of skeletal muscles. Second, the relative amplitude of vasomotion during haemorrhagic hypotension plotted against decreasing blood pressure exhibited a reverse U-shaped curve with a maximum at 40-60% of the control blood pressure, while the frequency of vasomotion remained relatively constant until the last stage of haemorrhage and centred around 3-5 cpm.