Introduction. Panic disorder (PD) and major depressive disorders (MDD) are serious mental disorders but the mechanisms underlying the pathophysiology are poorly understood. Nitric oxide (NO) is a gas considered to play an important role in mediating anxiety and stress response and is synthesised from nitric oxide synthase (NOS). The endothelial isoform (eNOS) has been found also in platelets. Homocysteine (Hcy) is an amino acid which naturally occurs in the human body. Elevated levels are linked to increased risk of cardiovascular, neurological and psychiatric diseases. In this study we aimed to evaluate NO, platelet aggregation and Hcy levels in PD and MDD patients. Materials and methods. Nineteen PD and 18 MDD patients participated in this study. NO levels were measured spectrophotometrically, platelet aggregation levels were measured in an aggregometer and Hcy levels were measured by HPLC. Results. NO levels were significantly lower in patients with MDD and PD than in control subjects (P < 0.05). Hcy and platelet aggregation levels were significantly higher in patients with MDD and PD than in control subjects (P < 0.05). Conclusion. Further more detailed studies are needed to find out the effects of drugs on these parameters or to disclose the exact mechanism underlying the alteration of these parameters.