The effects of morphine (CAS 57-27-2), nalbuphine (CAS 20594-83-6) and pentazocine (CAS 359-83-1) on gastric emptying of indigestible solids were studied. In rats, which were fasted for 24 h, either morphine, nalbuphine, pentazocine or saline was injected intraperitoneally, and ten steel balls (1.0 mm in diameter) were inserted into the stomach. At 3 h, the number of balls which had passed into the small intestine was counted. If these drugs inhibited gastric emptying, naloxone was injected concurrently to study the mechanisms of the inhibitory effect. Morphine, nalbuphine and pentazocine significantly inhibited gastric emptying (ED50:0.041 [95% confidence interval: 0.0078-0.14] mg kg-1, 0.0012 [0.00037-0.0081] mg kg-1 and 0.81 [0.41-1.30] mg kg-1, respectively). Naloxone 0.3 mg kg-1 antagonized the inhibitory effect of both morphine 1.3 mg kg-1 (ED75) and nalbuphine 0.005 mg kg-1 (ED75). In contrast, naloxone 0.3 or 1.0 mg kg-1 did not antagonize the effect of pentazocine 1.9 mg kg-1 (ED75), but a higher dose of naloxone (3.0 mg kg-1) did so. Therefore, in the rat, morphine, nalbuphine and pentazocine inhibit gastric emptying of indigestible solids, and it is likely that morphine and nalbuphine inhibit the emptying through the same opioid receptor, whereas pentazocine does so through a different receptor interaction.