The S-naproxen betainate sodium salt monohydrate (naproxen-betaNa, CAS 104124-26-7, Aprenin, test drug), and the sodium salt of S-naproxen (reference), were administered to twelve healthy volunteers of both sexes according to a crossover design, in a single dose of one 575 mg capsule of test, containing 342 mg of S-naproxen and two 275 mg tablets of reference, containing 502 mg of S-naproxen. Blood samples were drawn off over a 24-h period before (time 0) and after administration at foreseen time intervals. Naproxen was measured in plasma by a validated HPLC assay with UV detection which was able to detect 1 microgram/ml and proved to be linear in the range 1-100 micrograms/ml. The non-compartmental pharmacokinetic parameters obtained were statistically processed according to the EU guidance note on bioavailability and bioequivalence Cmax, AUC0-24h and AUC0-infinity were normalized to the dose of 502 mg of naproxen and log-transformed before statistical analysis to assess bioequivalence. Dose-normalized values of plasma concentrations encountered with the two formulations proved to overlap, with the exception of the first sampling time which showed naproxen concentrations that were higher with test drug than with reference. The specific test for bioequivalence led to 90% confidence intervals within the 80-125% range with target pharmacokinetic parameters, whereas the time to peak (tmax) observed with the test and reference drugs did not differ to any statistically significant degree when analysed with Wilcoxon's non-parametric test. It is concluded that the test drug should be declared bioequivalent with the reference drug in terms of dose-normalized concentrations, despite the more rapid increase in plasma concentrations of naproxen observed at the first sampling time with test drug.