Nitrofurantoin, a urinary tract antibacterial agent, was administered orally to human subiects in a therapeutic dose regimen for 7 days in microcrystalline form in a tablet or in macrocrystalline form in a capsule. The urine consistently contained substantial amounts of nitrofurantoin in each case, although the drug was not detectable in any of the blood samples collected during the study. Even though significantly greater urinary drug recoveries were obtained after the tablet than after the macrocrystalline drug, both formulations yielded adequate amounts of the drug in urine for potential antibacterial effectiveness. The differences in urinary drug recovery and in urinary drug‐excretion patterns between the two formulations suggests a slower rate of absorption of the macrocrystals in a capsule than of the microcrystalline form in a tablet.