This paper reports the data of nitrous oxide (N2O) environmental pollution in 269 operating rooms of 47 hospitals in Italy in 1989-91. In 40% of the operating rooms the N2O concentrations are lower than 50 ppm, limit value proposed by Health Council for new operating rooms. In 65.4% of the operating room studied, N2O mean environmental concentrations are lower than 100 ppm, value proposed by the above-mentioned Health Council as limit value for the already existing operating rooms. Concerning the biological monitoring, the authors report several N2O data in urine (2193), whose levels confirm the data obtained with environmental monitoring. The authors believe that they presently have reliable methods to perform biological and environmental monitoring: the two techniques are complementary in the assessment of the exposure. The method of measuring N2O concentrations as exposure index, both for the environmental and biological monitoring, is considered very useful to simplify the performance of the analyses. In order to assess exposure more precisely, it is however necessary also to determine the environmental and/or biological measure of the other different anaesthetics used.