Nitrofurantoin (50 mg) was administered in a three-way random crossover design to six healthy men. After a 45-min intravenous infusion the plasma concentration data could be described by a two-compartment open-body model with a terminal t 1/2 of 58.1 +/- 15 min. Oral availability of a tablet was 0.87 +/- 0.13 on a fasting stomach and 0.94 +/- 0.13 when taken with food. Although absorption appeared to be complete, the absorption rate profile was complex and erratic. Two subjects failed to achieve the minimum effective urine concentration of 32 micrograms/ml. After the intravenous infusion 47 +/- 13% of the dose was excreted unchanged in the urine and 1.2 +/- 0.3% was recovered as the reduced metabolite aminofurantoin.