The aim of this study was to investigate the action of opioids (the μ receptor agonist morphine) and the antagonist naloxone on inhibition of oxytocin release and milk let-down in response to milking in dairy cows. In the first experiment, cows were injected with 0, 21, 70 and 210 mg morphine 10 min before milking on four successive days. Plasma oxytocin levels after 1 min manual stimulation of the udder were reduced by 70 and 210 mg morphine, and milk let-down was inhibited at the latter dose. In the second experiment, cows were injected after a control milking with 210 mg morphine (or 350 mg at 10 min before milking the following day if not effective) to inhibit milk flow. On the following day the inhibiting dose of morphine was given with 210 mg naloxone. Naloxone injection given before morphine had no effect on plasma oxytocin concentrations, but abolished the inhibition of oxytocin release by morphine and potentiated oxytocin release in response to milking. Naloxone alone injected the day after control milking increased oxytocin levels during milking, suggesting involvement of the opioid system in milking. A model has been developed for the control of opioid effects during milking. Morphine suppressed oxytocin release during milking in a dose-dependent manner and the effect was reversible by naloxone.