The effects of tonazocine and zenazocine, two mixed agonist/antagonist analgesics, have been evaluated in a range of antinociceptive assays and in isolated tissue preparations in vitro. Both tonazocine and zenazocine were antinociceptive in writhing tests and in the i.a. bradykinin test, and were antagonists in the rat tail flick test. Additionally, zenazocine demonstrated some antinociceptive activity in the rat tail flick test. In vitro, both tonazocine and zenazocine demonstrated agonist and antagonist properties at mu receptors in the GPI and agonist properties at delta receptors in the MVD. In general, the agonist properties of zenazocine were more marked than those of tonazocine, and the antagonist properties of tonazocine were more marked than those of zenazocine.