FK 33–824, a potent enkephalin analogue was administered systemically, 0.5–5 mg/kg, to preterm neonatal rabbits. A marked decrease in respiratory frequency as well as irregular breathing and apneic spells was recorded at 5 mg/kg. Tidal volume was not affected. The enkephalin-induced respiratory depression was restored immediately after naloxone (10 mg/kg). Based on the results it may be speculated upon whether brain stem enkephalin neurons are involved in the pathogenesis of neonatal transient apnea and irregular breathing.