Bioequivalence test of $Asthcontin^{circledR}$ tablet, a commercial slow-release theophylline (TP) dosage form, was performed using $Slo-bid^{circledR}$ capsule as the reference. Since it has been confirmed that the saliva concentration of TP is closely correlated with the plasma concentration in man, the area under the saliva concentration-time curve was used as a bioavailability parameter. The statistical analysis showed that the two dosage forms are equivalent in bioavailability estimating from the saliva concentration. The results supported that the use of soliva as a test sample provides simple and easy techniques for bioequivalence tests of TP-containing dosage forms.