Male Wistar rats were injected epidurally with various doses of sufentanil in 3 different dilution volumes, in order to determine differences in the lowest ED50 values for analgesia and side-effects. The lowest ED50 for both a TWR latency greater than 6 and greater than or equal to 10 s was significantly lower in the 20-microliters group as compared to the 2-microliters group. With an injection of 5 microliters, intermediate values were obtained. With regard to the side-effects, there was a trend towards increased systemic resorption with larger diluent volumes in spite of a better and more selective spinal effect, as evidenced by increasing specificity ratios. In terms of duration of analgesia at fixed doses of sufentanil, different results were obtained. At low doses of sufentanil, a pronounced diluent volume effect was present and the duration of analgesia increased as a function of the volume. At higher doses of sufentanil, however, the volume effect faded away and no differences were observed as a function of the diluent.