Summary. Six lactating sows were injected through an indwelling vena cava cannula with naloxone (2·5 mg/kg body weight) on Day 15 post partum. Blood samples were collected through the cannulas at 10-min intervals for 8 h before and 10 h after naloxone administration. Plasma prolactin and LH concentrations were measured by radioimmunoassay. Naloxone caused a marked suppression of plasma prolactin concentrations lasting 4–6 h. LH concentrations were also affected by naloxone: LH rose to reach maximum values 20–50 min after naloxone treatment. Pretreatment values were recorded 200–300 min after the treatment. These results indicate that endogenous opioids are involved in causing the endocrine patterns occurring during lactation, i.e. high prolactin and low LH concentrations.