Background & purpose: There are occupational hazards related to Nitrous Oxide (N2O) in hospitals operating and recovery rooms. These hazards include the decrease of mental performance and audio-visual ability, and reduced fertility, spontaneous abortion and neurological, renal and liver diseases. In this survey, the concentration of Nitrous Oxide in indoor air of hospitals operating and recovery rooms in Babol University of medical sciences was determined. Materials & Methods: This descriptive study was performed in 23 operating rooms and 3 recovery rooms in 3 educational hospitals of Babol medical sciences university. The rooms with continuous usage of N2O with 2-3 lit/min of flow of general anesthesia were studied. For sampling and detecting N2O concentration as part per million (ppm), a portable IR spectrophotometer (3015 model of Bacharach Inc.) was used. The sampling was performed in 5 different zones of the operating rooms and 1zone of recovery room in 3 different hours of work time (8:30-9AM, 10:30-11AM and 12:30-1:00PM). One-way ANOVA ,SPSS 18 was used to analyze data and comparing the means. Results: N2O concentration mean in 5 different zones of the operating rooms was 318± 22.6, 325.5± 24.1, 299± 21.8, and 301± 22, 314± 23.7 ppm and in recovery room, it was 51± 15 ppm. There was no significant difference between the means of N2O concentration in different zones of the operating rooms, but the means of N2O concentration in different zones of the operating rooms and recovery room were significant (p<0.05). Conclusion: Considering high average concentration of Nitrous Oxide in different operating and recovery rooms with maximum contamination levels of N2O, this situation subjects the health personnel to risk. Therefore, further research and applying protection utilities are recommended.