['To investigate the influence of co-administrated Da-Chaihu-Tang (DCT; a traditional Chinese formulation) on the pharmacokinetics of nifedipine, as well as the safe optimal dosing interval to avoid the adverse interactions.', 'A single dose of DCT was administered with nifedipine simultaneously, 2\u2009h before, 30\u2009min before or 30\u2009min after nifedipine administration. Pharmacokinetics of nifedipine with or without DCT were compared. The influences of DCT on nifedipine intestinal mucosal and hepatic metabolism were studied by using rat in-vitro everted jejunal sac model and hepatic microsomes.', 'A simultaneous co-administration of DCT significantly increased the area under concentration-time curve from time zero to infinity (AUC0-inf ) of nifedipine. In-vitro mechanism investigations revealed that DCT inhibited both the intestinal and the hepatic metabolism of nifedipine. Further study on the optimal dosing interval for nifedipine and DCT revealed that administration of DCT 30\u2009min before or after nifedipine did not significantly change the AUC of nifedipine.'] Does da-Chaihu-Tang alter the pharmacokinetics of nifedipine in rats and a treatment regimen to avoid this?