Nitroglycerin was administered to eight healthy volunteers in the form of sublingual tablets, oral sustained-release tablets, and an oral solution. Blood samples were collected for measurement of nitroglycerin and its two isomeric glyceryl dinitrate metabolites. Blood pressure and pulse rate were monitored; subjective evaluations of headache, dizziness, facial flushing, skin irritation, and gastrointestinal upset were made. Nitroglycerin itself was virtually undetectable after the solution and tablet preparations; the metabolites were consistently detectable from a few minutes after dosing to 24 h later. Mean total (nitroglycerin plus metabolite) concentrations were comparable in the 15 min following sublingual administration, and the 8 h following tablet administration. The relative bioavailability of the tablets in comparison with the oral solution was 70 per cent based on metabolite concentrations. Nitroglycerin sustained-release tablets appear to exert their beneficial effects in the prolonged prophylaxis of angina through active metabolites.