Objectives: This paper reports on the trend of the stressogenic stimulus caused by a repeated exposure to 1-methylnaphthalene (1-MN) vapors at the nominal concentrations of 0 mg/m³ (the control restrainer), 50 mg/m³ or 200 mg/m³ in the nose-only inhalation system, by analyzing the serum corticosterone (CORT) levels in rats. Material and Methods: Three groups of rats were exposed in restrainers to 1-MN vapors at the nominal concentrations of 0 mg/m³, 50 mg/m³ or 200 mg/m³ for 5 days. One control group of animals spent all the time during the experiment in an individually ventilated plastic cage. The serum CORT concentrations were determined in all 4 groups of the rats. The blood samples drawn from the tail vein were collected every day after termination of the 6-h exposure. On the fifth day, blood samples were collected 15 min, 30 min, 45 min, 1 h, and 3 h after termination of the 6-h exposure. Results: On the fifth day of the study, no statistically significant changes in body weights between all groups of animals were found. After 5 days of the observation, increased food intake in the control groups was noted. Significantly higher CORT concentrations in the rats exposed to 1-MN at 200 mg/m³ and in the animals from the control restrainer were found, comparing to the animals exposed to 1-MN at 50 mg/m³ and the animals from the control cage. Conclusions: The application of 6-h restraining induced high concentrations of the stress hormone, CORT, in the blood of rats. The short-term exposure of rats to 1-MN non-linearly reduced the restraint stress measured with CORT concentration. Int J Occup Med Environ Health. 2020;33(5):691–9