Increases in pain thresholds have been observed during gestation in a variety of laboratory animals and humans. The specific events that occur during pregnancy that are responsible for this increase in maternal pain threshold are not known. In order to investigate whether or not the changes in the peripheral concentration of sex steroids that are known to occur during gestation contribute to the analgesia of pregnancy, the effect of pseudopregnancy on jump thresholds was determined. Pseudopregnancy is associated with a significant increase in the mean jump threshold. In contrast, rats treated chronically with the opioid antagonist naltrexone do not manifest an increase in jump threshold during the pseudopregnant condition. These results suggest that peripheral sex steroids whose peripheral concentration profile is altered during pseudopregnancy might be important modulators of opioid systems that regulate responsiveness to aversive stimuli.