The in vivo absorption of crystalline nitrofurantoin and the dosage forms of nitrofurantoin prepared with microcapsules were carried out in man by determination of urinary excretion of unchanged nitrofurantoin. The cumulative amount of drug excreted and the duration of the therapeutic urine levels were compared. The microcapsule administration showed that the peak reached during the excretion of nitrofurantoin in urine, decreased significantly when compared to the pure drug. This could be an explanation for the decrease in side-effects of nitrofurantoin such as nausea and vomiting. Experiments in male albino rats showed that the microcapsules did not produce gastric haemorrhage seen with the same doses of the pure drug.