We evaluated the effects of σ1 receptor inhibition on μ-opioid-induced mechanical antinociception and constipation. σ1-knockout mice exhibited marked mechanical antinociception in response to several μ-opioid analgesics (fentanyl, oxycodone, morphine, buprenorphine and tramadol) at systemic (subcutaneous, s.c.) doses that were inactive in wild-type mice, and even unmasked the antinociceptive effects of the peripheral μ-opioid agonist loperamide. Similarly, systemic (s.c.) or local (intraplantar) treatment of wild-type mice with the selective σ1 antagonists BD-1063 or S1RA potentiated μ-opioid antinociception; these effects were fully reversed by the σ1 agonist PRE-084, showing the selectivity of the pharmacological approach. The μ-opioid antinociception potentiated by σ1 inhibition (by σ1 receptor knockout or σ1 pharmacological antagonism) was more sensitive to the peripherally-restricted opioid antagonist naloxone methiodide than opioid antinociception under normal conditions, indicating a key role for peripheral opioid receptors in the enhanced antinociception. Direct interaction between the opioid drugs and σ1 receptor cannot account for our results, since the former lacked affinity for σ1 receptors (labeled with [(3)H](+)-pentazocine). A peripheral role for σ1 receptors was also supported by their higher density (western blot results) in peripheral nervous tissue (dorsal root ganglia) than in several central areas involved in opioid antinociception (dorsal spinal cord, basolateral amygdala, periaqueductal gray and rostroventral medulla). In contrast to its effects on nociception, σ1 receptor inhibition did not alter fentanyl- or loperamide-induced constipation, a peripherally-mediated nonanalgesic opioid effect. Therefore, σ1-receptor inhibition may be used as a systemic or local adjuvant to enhance peripheral μ-opioid analgesia without affecting opioid-induced constipation.