We determined the blood concentrations of caffeine (CA) and its three primary dimethylxanthine metabolites: theobromine (TB), paraxanthine (PX), and theophylline (TP), and trimethadione (TMO) and its demethylated metabolite dimethadione (DMO) after the simultaneous administration of CA (10 mg kg-1) and TMO (4 mg kg-1) to rats pretreated with phenobarbital (PB:40 and 80 mg kg-1 i.p. daily for 3 days) and 3-methylcholanthrene (MC: 10 and 20 mg kg-1 i.p. daily for 2 days). After the oral administration of CA and TMO, the PB-pretreated rats showed a significant increase in TMO metabolism, whereas the CA metabolism was greatly accelerated in rats pretreated with MC. In five pretreated groups, there were correlations which were determined 1 h after the administration of CA and TMO, between the plasma half-life (t1/2) of CA and the TB/CA, PX/CA, and TP/CA ratios. The coefficients of correlation (r) ranged from -0.881 to -0.908, and the coefficients of correlation between the CL of CA and the TB/CA, PX/CA, and TP/CA ratios ranged from 0.959 to 0.989. There were high correlations between the t1/2 of TMO and the DMO/TMO ratio at 1 h after administration with r = -0.966, and between the CL of TMO and the DMO/TMO ratio with r = 0.971. The above results suggest that one blood sampling after the simultaneous administration of CA and TMO enables prediction of the degree of each hepatic drug-oxidizing activity because the P-450 isozymes involved in metabolism of CA and TMO are different.