Pharmacokinetics of mequitazine, a recently introduced peripheral H1-histamine receptor antagonist of phenothiazine type, was followed up to 72 h after the single oral dose of 5 mg of the drug to eight fasted healthy volunteers. Each subject was treated thrice with a dosing interval of 15 days or more. Thus all the results were triplicated. Serum mequitazine was measured by mass fragmentography using a gas-liquid chromatograph/mass spectrometer set in the electron impact mode. Urine phenothiazines were determined fluorometrically before and after cleaving phenothiazines from their glucuronide conjugates. Peak concentration of mequitazine in serum was 3.19 +/- 1.70 (s.d.) ng.ml-1, time to peak concentration 5.67 +/- 1.68 h, elimination half-life 45 +/- 26 h, and elimination rate constant 0.018 +/- 0.007 h-1. Only 10.9 +/- 3.3% of the dose appeared in urine in unconjugated plus the glucuronidated form during the first 72 h. About 46% of the urinary phenothiazines were glucuronide conjugates. The results suggested that after the oral administration only low mequitazine concentrations appeared in serum, most of the drug seemed to be deactivated by the extrarenal route, and the kinetic properties of the drug resembled those of several phenothiazines used for psychiatric therapy.