Ciprofloxacin distribution in muscle, subcutaneous fat and perirenal and perivesical fat was studied following a single i.v. bolus injection of 100 mg or oral administration of 500 mg. Levels in muscle were on average 0.75 mg/kg; diffusion into muscle was rapid, whereas elimination from muscle was slow. Similar peak levels were recorded in fatty tissues. However, penetration into subcutaneous fat in particular may be delayed in individual cases. Following an initial lagphase of up to one hour after i.v. injection, ciprofloxacin distribution was as efficient in these patients as in the others. Tissue levels following oral administration were monitored 12 hours after intake. On average, ciprofloxacin concentrations in serum, muscle and perirenal fat were 0.17 mg/l, 0.20 mg/kg and 0.11 mg/kg, respectively. Thus, ciprofloxacin is distributed effectively throughout the extravascular space following i.v. as well as oral administration.