Pyrazinamide (PZN) was administered to 10 patients of pulmonary tuberculosis (for 7 consecutive days) each day after an overnight fast. On 8th day serum levels and urinary elimination were measured at 2,4,6 and 8 hours. Simultaneous administration of isoniazid to same patients significantly decreased the peak serum concentration (Cmax). Although, time to peak serum concentration (Tmax) remained unaffected, serum half life (t1/2) prolonged, the elimination rate constant (Kel) and area under serum concentration time curve (AUC) decreased and apparent volume of distribution (Vd) and plasma clearance (Clp) of PZN increased significantly. However, the cumulative per cent dose of PZN excreted in urine was not changed significantly. Although, serum levels of PZN were decreased at 2, 4, 6, and 8 hours, PZN levels remained above minimum effective concentration thereby not affecting the therapeutic status of PZN administered in combination with isoniazid, if PZN is administered in moderate doses.