It has been proposed that nitric oxide (NO) induced headache in primary headaches may be associated with release of calcitonin gene-related peptide (CGRP). In the present study we aimed to investigate plasma levels of CGRP during headache induced by the NO donor glyceryl trinitrate (GTN) in 16 patients with chronic tension-type headache and 16 healthy controls. The subjects were randomly allocated to receive 0.5 microg/kg/min GTN or placebo over 20 min on two headache-free days. Blood samples were collected at baseline, 10, 20 and 60 min after start of infusion. Both patients and controls developed significantly stronger immediate headache on the GTN day than on the placebo day and the headache was significantly more pronounced in patients than in controls. There was no difference between the area under the CGRP curve (AUCCGRP) on GTN vs. placebo day in either patients (P=0.65) or controls (P=0.48). The AUCCGRP recorded on the GTN day did not differ between patients and controls (P=0.36). Both in patients and controls, CGRP levels changed significantly over time, on both the GTN and placebo days (P < 0.05). The present study indicates that NO-induced immediate headache is not associated with release of CGRP.