Dextromethorphan is an N-methyl-D-aspartate (NMDA) noncompetitive antagonist which has been used as an antitussive, analgesic adjunct, probe drug, experimentally to attenuate acute opiate and ethanol withdrawal, and as an anticonvulsant. A metabolite of dextromethorphan, dextrorphan, has been shown to behave pharmacodynamically in a similar manner to dextromethorphan. The pharmacokinetics of dextromethorphan were examined in six healthy dogs following intravenous (2.2 mg/kg) and oral (5 mg/kg) administration in a randomized crossover design. Dextromethorphan behaved in a similar manner to other NMDA antagonists upon injection causing muscle rigidity, ataxia to recumbency, sedation, urination, and ptyalism which resolved within 90 min. One dog repeatedly vomited upon oral administration and was excluded from oral analysis. Mean +/- SD values for half-life, apparent volume of distribution, and clearance after i.v. administration were 2.0 +/-0.6 h, 5.1 +/- 2.6 L/kg, and 33.8 +/- 16.5 mL/min/kg. Oral bioavailability was 11% as calculated from naive pooled data. Free dextrorphan was not detected in any plasma sample, however enzymatic treatment of plasma with glucuronidase released both dextromethorphan and dextrorphan indicating that conjugation is a metabolic route. The short half-life, rapid clearance, and poor bioavailability of dextromethorphan limit its potential use as a chronic orally administered therapeutic.