Objective: To explore the effects of Nigella sativa oil (NSO) on the histopathological and biochemical changes that inhaled formaldehyde (FA) induces on the testicular tissue of rats. Methods: Thirty three adult male rats were separated into five groups as follows: C, the control group; 4FA group which received FA for 4 weeks; 13FA group which was given FA for 13 weeks; 4FA+NSO group which was administered FA plus NSO for 4 weeks; 13FA+NSO group which was treated with FA plus NSO for 13 weeks. FA was administered through inhalation for 8 h 5 days a week at a dose of 5 ppm in a special glass cage, and NSO was administered orally 1 mL/kg once daily. Rats were decapitated at the end of the experiment and testicular tissue specimens were harvested for histopathologic and biochemical assessment. Results: Compared to the C group, reduction was observed in the number of intact tubules and in the mean germinative epithelium thickness of the FA groups. Significant increase was observed in the number of intact tubules with the long-term (13 weeks) administration of NSO together with FA. Reduced glutathione peroxidase activity was found and oxidative stress index values were measured higher in the 4FA and 13FA groups versus the C group (P<0.05). Moreover, total antioxidant status levels decreased only in the 4FA group (P<0.05) while only the 13FA group significantly increased malondialdehyde levels and reduced catalase activities in comparison with the C group. In the 13FA+NSO group, malondialdehyde levels decreased however glutathione peroxidase and catalase activities increased compared to the 13FA group. Differences measured in total antioxidant status levels were found to be statistically significant only between the 4FA and the 4FA+NSO groups. Conclusions: NSO as an antioxidant should be used for a longer term to achieve protective efficacy both histoparhologically and biochemically in the testicular tissue.