An internal solid contact sensor (ISCS) for the determination of methacycline hydrochloride (MC·Cl), Pt/PPy/PVC(MC-PT), is described, based on the use of conducting poly(pyrrole) (PPy) as solid contact material and MC-phosphotungstate (PT) as the ion exchanger and dibutyl phthalate (DBP) as the plasticizer. A direct potentiometric method for the quantitative analysis of MC·Cl is also described. Under the condition of pH 2.7, the linear concentration range, slope (25°C) and detection limit of the sensor are 6.4 × 1.0^-6 - 3.0 × 1.0^-3 M, 52.4 ± 0.2 mV/decade and 4.4 × 1.0^-6 M, respectively. The response time is <5 s. The determinations of MC·Cl in tablets were carried out by direct potentiometry. The average recovery and relative standard deviation are 100.1 and 0.7% (n = 4), respectively.