This single-blind crossover study compared the human bioavailability of macrocrystalline nitrofurantoin (Furadantine MC) and two prolonged-action hydroxymethylnitrofurantoin formulations (Urfadyn PL, bid, and Uridurine, tid), based on plasma nitrofurantoin concentrations and urinary nitrofurantoin excretion. The drugs were administered to 16 healthy females for a single day according to the recommended daily dosages. For comparison, Furadantine MC was administered both at the qid dosage recommended by the manufacturer and at tid dosage. Pharmacokinetic parameters determined were maximum plasma concentration after first dose, minimum plasma concentration after first dose, area under the plasma concentration-time curve (AUC), cumulative renal excretion over 30 hours (ARE), overall renal clearance, total body clearance, and bioavailability relative to Furadantine MC qid, based on plasma AUC (F) and ARE (Fren). F for Furadantine MC 100 mg tid was 108 +/- 25 percent (mean +/- SD); for Uridurine 100 mg tid and Urfadyn PL 100 mg bid, F equalled 86 +/- 33 percent and 53 +/- 20 percent (p less than 0.05), respectively. A similar relationship was observed between Fren for Furadantine MC 100 mg qid and the respective Fren of Furadantine MC 100 mg tid, Uridurine 100 mg tid, and Urfadyn PL 100 mg bid. No significant difference was found between the respective F and Fren of each of the drugs studied. Although bioavailability was comparable for Furadantine MC tid and qid, the single-day design of these studies precludes inferring that these dosage schedules are therapeutically equivalent. However, the significantly lower relative bioavailabilities for the prolonged-action hydroxymethylnitrofurantoin formulations suggest that Urfadyn PL 100 mg bid and Uridurine 100 mg tid are not pharmacokinetically equivalent to Furadantine MC.