Background: ANF is a potent diuretic, natriuretic and vasorelaxant hormone. The objective of the present study was to examine the effect of opioid receptor stimulation by morphine after surgery on endogenous ANF production and diuresis. Methods: Prospectively, 11 women undergoing surgery for either uterine leiomyomas, chronic pelvic discomfort or desire for definitive contraception by laparotomy were evaluated. Venous samples were collected at fixed times. Concentrations of ANF were measured by commercially available radioimmunassay test kits. Statistical analysis was performed by the Friedman Two way ANOVA, Kruskal-Wallis 1-way ANOVA and Mann-Whitney U-Wilcoxon Rank Sum W Test. The level of significance was set at probability below 0.05. Results: There were statistically significant changes in the serum levels of ANF (p = 0.0028), in pain score (p < 0.000 l) and urinary flow rate (p < 0.0001) after operation, while the diastolic (p = 0.0671) and systolic (p = 0.0543) blood pressure showed slightly significant changes. Conclusion: Our results show that i.v. administered morphine induces a potent diuretic effect via activation of opioid receptors and suggest that this effect is due to the enhanced release of ANF. However the mechanism by which morphine induces the ANF release remains to be evaluated.