This study was done to determine if chronic intermittent in utero exposure to morphine (MOR) during the last half of gestation results in hypoventilation and decreased ventilatory sensitivity to CO₂ in the neonate. Pregnant guinea pigs were randomly assigned to once-daily treatment with saline and 1.5, 5.0, or 15.0mg/kg MOR. Neonates were studied for 3 weeks. Prenatal exposure to 5.0 and 15.0 mg/kg MOR significantly increased neonatal minute ventilation and central inspiratory drive on day 7 while breathing room air or 5% CO₂. The increase in minute ventilation was part of a withdrawal syndrome that included increased locomotor activity, but was not due to an increase in metabolic rate or sensitivity to CO₂. We conclude that neonatal guinea pigs exposed once daily to MOR during the last half of gestation hyperventilate during the 1 st week after birth. These changes are neither permanent nor followed by hypoventilation or depressed sensitivity to CO₂.