Patients with pulmonary arterial hypertension (PAH) are currently treated with more than one drug. Sildenafil, a phosphodiesterase type 5 (PDE-5) inhibitor, and rosiglitazone, a peroxisome proliferator-activated receptor γ (PPAR-γ) activator, is one of those combinations that could be used in PAH. To monitor the pharmacokinetics of sildenafil in the presence of rosiglitazone, we have developed and validated a sensitive, specific and rapid liquid chromatography-tandem mass spectrometric (LC–MS/MS) method. We have used this validated method to study the pharmacokinetics of sildenafil and rosiglitazone after intravenous administration of sildenafil alone or a combination of sildenafil plus rosiglitazone to adult male Sprague-Dawley rats. Sildenafil and rosiglitazone were extracted from plasma by protein precipitation with methanol. With an octadeuterated sildenafil as the internal standard, the drugs were separated via gradient elution using a C18 column and formic acid in methanol or in water as the mobile phase with a flow rate of 0.25 mL/min. Both sildenafil and rosiglitazone samples in rat plasma produced linear response, when the concentration ranged between 5 and 1000 ng/mL (r2 > 0.99). The pharmacokinetics study suggests that intravenous co-administration rosiglitazone plus sildenafil increases the plasma concentration of sildenafil and extends the drug's elimination half-life.