The loss of nitroglycerin (glycerol trinitrate (GTN), Perlinganit) when infused at various rates through polyvinyl chloride (PVC) tubing was measured against time by continuously monitoring the GTN concentrations at the outflow of the tubing using a direct photometric method. The portion of GTN loss depends on the rate of infusion; at flow rates of 3-6 ml/h, GTN losses of up to 60% may be expected. Polyethylene (PE) tubing does not show this reduction in the outflow concentration of GTN. When GTN is infused through PVC tubing, the dosage cannot be controlled and any assessment of pharmacokinetics or absolute bioavailability becomes invalid. This effect of tubing accounts for apparent losses of bioavailability of up to 50%. Other organic nitrates like isosorbide dinitrate are shown to be absorbed into PVC in a similar way. The losses increase with larger liphophilicity of the drug.