This study was carried out for detection of nitric oxide (NO) levels in healthy (Group 1, n:78) and infected (Group 2, n:63) quarters of pregnant heifers. The mean nitric oxide (NO) concentrations were calculated of about 15,58 +/- 2,21 mu M/ml and 42,69 +/- 6,03 mu M/ml for Group I and Group 2, respectively. A significant difference was found between two groups, statistically (p<0,001). The mean NO concentrations were compared between quarters infected with Staphylococcus aureus (S. aureus) and coagulase negative staphylococcus (CNS) in Group 2 and measured to be; 38,50 +/- 8,47 mu M/m +/- and 44,17 +/- 7,71 mu M/ml, respectively (p>0,05). Additionally, lacteal secretions were classified to be thickened and watery according to viscosity. Intramammarial infection rates (IMI) and NO concentrations were compared in low (watery) and high (thickened) viscosity samples. Risk of IMI was found higher in thickened samples than watery (p<0,001). Every 1 unit (mu M/ml) increase in the NO concentration boosted the risk of IMI 1,036-fold (p<0,001).