The aetiology of anal fissure is unclear, but there is an association with high maximum resting pressure (MRP). Internal sphincterotomy reduces MRP and heals fissure through an increase in local blood supply. Glyceryl trinitrate (GTN) is a nitric oxide donor which contributes to internal anal sphincter relaxation via a non-adrenergic non-cholinergic pathway. GTN ointment was applied topically in different concentration to the anal margin in patients with chronic anal fissure to monitor its effect primarily on MRP and secondarily on fissure healing. Nineteen patients with chronic anal fissure were treated with ointment containing increasing concentrations of GTN (0.2-0.8 per cent) to produce a reduction in MRP of greater than 25 per cent. The actual dose of GTN varied as no standard delivery system has been developed, but a 'typical amount' of GTN ointment weighed about 200 mg. In 15 of 19 patients, a concentration greater than 0.2 per cent was required to lower the MRP by at least 25 per cent. The minimum concentration of GTN that reduced the resting pressure by at least 25 per cent was prescribed and local application was carried out by the patient twice daily for 6 weeks. At 6 weeks, nine patients had healed, six required sphincterotomy and four were lost to follow-up. Eight of the nine patients with healed fistula required a GTN concentration of 0.3 per cent or more. Sixteen patients were resistant to the usually effective does of 0.2 per cent GTN. In three there was tachyphylaxis and the duration of action of GTN was less than the 12 h described previously in control patients. Two patients did not fulfil the study because of headache.