Side effects of nitroaromatics used as drugs are often assumed to be caused by incomplete enzymatic reduction of the nitro group. However, photoactivation, although usually not considered as a cause of the toxic effects of nitroaromatics, can play a considerable role. Nifedipine, a nitroaromatic as well, is an important drug used in the treatment of myocardial ischaemia and hypertension. It is extremely sensitive to ultraviolet and visible light up to 450 nm and forms a nitroso derivative in vitro. In the present study it is shown that the nitroso compound is a short-lived intermediate in blood and plasma. It reacts with other constituents to form a stable lactam. In vivo, this lactam proves to be rapidly cleared from the blood of rats and is excreted almost quantitatively via the bile. The first photoproduct of nifedipine, nitroso nifedipine, is shown to be converted into the lactam mentioned. Beside the lactam, two other products, which are considered to be derivatives of the lactam, are found. One of these two products is also found in the bile of a rat that was exposed to UVA light after intravenous nifedipine administration. This shows that in an in vivo situation, photoproducts can reach organs other than those exposed to the light. In vitro about 45 to 50% of the original amount of nifedipine is not recovered after exposure of nifedipine to UVA in the presence of bovine serum albumin or after incubation of nitroso nifedipine with bovine serum albumin in the dark. As complex binding of nifedipine to plasma proteins is high, the latter finding may have important implications for the situation in vivo.