Nitroglycerin-(NTG)-induced headache and dilatation of the radial artery were followed in a double blind, randomized, placebo-controlled, cross-over study in 6 healthy volunteers. NTG 0.5 microgram.kg-1 x min-1 or saline were infused i.v. for 7 h, and subsequently the infusion rate was doubled for 10 min. The radial artery diameter was measured repeatedly with high frequency ultrasound and pain was scored using a 10 point verbal scale. After 5 min of NTG infusion both headache and the arterial diameter differed significantly from baseline, and no further significant change occurred. The intensity of the headache was mild to medium (median headache score 3, range 1-7). The mean dilatation of the radial artery was 36%. The dilatation in each individual, was stable over time, both during NTG and placebo, and it did not change with the double infusion rate. The headache score in each individual was more fluctuant. No tolerance either to the NTG-induced headache or arterial dilatation was observed.