This study investigates the influence of possible stress due to housing in Bolman cages on antinociception and on respiratory depression following opioid administration. To evaluate the functional role of this stressor and to modulate it, rats were subcutaneously pretreated with the anxiolytic chlordiazepoxide (CDP; 10 mg/kg) or saline (SAL) before the immobilization in the Bolman cages and before the intravenous administration of small doses of morphine (MOR), sufentanil (SUF), or vehicle (VEH). Antinociception, respiratory impairment and stress were evaluated by means of the tail-flick latency, blood gas analysis, and serum corticosterone (CS), adrenocorticotropic hormone (ACTH), and prolactin (PRL) determinations. The results demonstrated that 10 mg/kg CDP did not alter the antinociceptive effects of low doses of morphine and sufentanil. CDP pretreatment differentially affected the various blood gas parameters. Compared to vehicle pretreatment, there was a larger decrease in PaO2 following MOR and SUF in the CDP-pretreated rats. The effects were most pronounced at the lowest doses of both opioids. A CDP potentiation was also observed for the short-lasting raises in PaCO2 with the lowest concentrations of the opioids. At higher concentrations of the opioids, CDP was without any effect. With regard to the stress hormones, immobilization and an intravenous injection resulted in increases in CS and PRL in both CDP- and VEH-pretreated rats. ACTH did not change in these controls. SUF prevented the CS raises independent of a CDP pretreatment, while ACTH only increased in the SUF plus CDP groups, pointing to a stress-reducing effect of SUF. Also, MOR without CDP prevented the increases in CS, but the opioid intrinsically increased ACTH. These results indicate that restraint in Bolman cages in the present setup, with animals recovering for several hours in these cages after being equipped with an arterial catheter, is stressful but without any significant effect on the opioid-induced antinociception. Pretreatment with an anxiolytic benzodiazepine only minimally affected the outcome of the opioids on respiratory depression and pointed to a stress-reducing effect of low doses of the opioids, especially sufentanil.