The aim of this study was to determine the role of nitrous oxide $N_{2}O$ in the bacterial growth in vitro. Methods: We assessed the effects of $N_{2}O$ on Escherichia coli (Ecoli) growth in a liquid nutrient broth under conditions similar to those in clinical practice, using an anaesthetic circuit. Bacterial inoculums were prepared from reference strains with exponential growth of $10^3$colony forming units (CFU) ml-] Sterile Petri dishes were filled with 10 ml suspension of an E coli strain and incubated at 37$circ$C Each strain was studied with or without exposure to 50% $N_{2}O$, using an airtight glass chamber in different exposure times (baseline, I and 3 h). For each time, a control point was obtained. Serial dilutions and agar plates were made and developed colonies were counted by using the standardized microbiological method'. Results: E coli was grown on nutrient agar after exposure to a mixture of NiJ-oxygen, and to air alone as the control group. Although there were no significant differences between the $N_{2}O$and the control group, the significant increases in bacterial growth were observed at the first and third hours in both groups (p 0.001). Conclusion: We concluded that $N_{2}O$ did not alter the growth of E.coli in vitro.