Abstract—The photobiological activity of nitrazepam, the active ingredient of the drug Mogadon®. which is known to induce phototoxic effects, that of clonazepam, the active ingredient of the drug Rivotril® and that of flunitrazepam, the active ingredient of the drug Rohypnol®, has been investigated. UsingSalmonella typhimuriumstrain TA‐100 as the test organism for cytotoxicity, it was demonstrated that not only nitrazepam, but also the structurally related compounds clonazepam and flunitrazepam were phototoxic. Furthermore, it was shown that this phototoxic effect was due to a photodynamic action of the compounds. Investigations of the (photo)biological activity of the decomposition products (7‐nitroso, 7‐hydroxylamino, 7‐amino analogues and the azoxy dimer) of the parent 7‐nitro compounds revealed that they were not involved in the phototoxic effect.Investigations on structurally related benzodiazepines have established that a 7‐nitro substituent in the benzodiazepine nucleus is the structural characteristic responsible for the appearance of phototoxicity and that the magnitude of this phototoxic effect is strongly determined by the presence or absence of a methyl‐group at the N‐1 position.Finally, photomutagenicity tests performed with the same bacterial indicator indicated that under the present experimental conditions the oxygen dependent reactive species formed upon irradiation of nitrazepam, clonazepam and flunitrazepam with longwave UV light did not exe