Effect of ulcerative colitis and smoking on rectal blood flow. Rectal blood flow was measured by laser doppler flowmetry over 60 minutes in eight patients with colitis in remission and eight healthy male non-smokers. Ten smokers were also examined on two occasions, one of which included smoking a cigarette. Plasma nicotine concentrations were measured in smokers. All subjects showed a pronounced fall in rectal blood flow in the first 30 minutes and patients with colitis had significantly higher values compared with smokers (p less than 0.002; p less than 0.04) and non-smokers (p less than 0.007; p less than 0.002) during the first and second 30 minutes respectively. Values in smokers and non-smokers were similar, but smoking a cigarette was associated with a significant fall in blood flow (p less than 0.04) and this change was inversely related to the rise in plasma nicotine concentration (r = -0.63; p less than 0.05). The findings may be relevant to the association between colitis and the smoking history. 