Aim: To investigate the influence of omeprazole on the pharmacokinetics of domperidone given as free base and maleate salt. Methods: An open, randomized, 2-period crossover study with a washout period of 7 d was conducted in 10 healthy Chinese, male patients. In each study period, the patients were adminis-tered a single oral dose of l0 mg domperidone as free base or maleate salt on d 1,20 mg omeprazole twice daily on d 2 and 3, and once on d 4. A single dose of 10 mg domperidone as free base or maleate salt was taken at 4 h after administration of omeprazole on d 4. Plasma samples were collected on d 1 and 4 after the adminis- tration of domperidone, and the plasma concentrations of domperidone were de- termined by a sensitive liquid chromatography-tandem mass spectrometry method.Results: For free-base domperidone, pretreatment with omeprazole resulted in a 16% decrease in maximum concentration (Cmax), compared with administration alone (P<0.05). However, for maleate salt, with the exception of an increase in t1/2,no pharmacokinetic parameters were significantly changed. When the free base and maleate salt were administered alone, no differences were found in any param-eters between the 2 formulations. In contrast, when they were administered in the presence of omeprazole, the Cmax of domperidone given as free base was lower (25.9%) than that given as maleate salt (P<0.05). Conclusion: Pretreatment of omeprazole does not affect the absorption of domperidone maleate, but leads to a moderately decreased rate of absorption of the free base.