Fluoroquinolone derivatives interact with methylxanthines (theophylline, caffeine) and metallic ion-containing drugs to different degrees. The rat appears to be a suitable model for predicting such interactions in man. It has been possible to determine the relationship between the chemical structure of the fluoroquinolone and the magnitude of the interaction. Fluoroquinolones with a bulky substituent at the position 8, such as sparfloxacin, lomefloxacin and fieroxacin, are less prone to interact with theophylline than those without an 8-substituent, such as enoxacin. This substituent determines the planarity of the whole fluoroquinolone molecule and the interaction tends to be more significant for planar fluoroquinolones. Furthermore, a 4'-nitrogen atom in the 7-piperazinyl group is essential for the interaction to occur. The nitrogen atom is possibly the site that binds cytochrome P-450, which catalyses theophylline metabolism. The reduction in bioavailability of fluoroquinolones by concurrent administration of aluminium hydroxide is more striking for derivatives with fewer substituents on the essential structure and on the piperazinyl group, such as norfloxacin, ciprofloxacin and enoxacin. Substitution at the 5-position diminishes the interaction, which suggests that the 5-substituent may affect the formation and/or stability of unabsorbable chelate complex which is the probable cause of the interaction. These findings are potentially useful in designing fluoroquinolones less prone to drug interactions.