1. Membrane potential (Vm) and resistance (Rm) of ventral respiratory group (VRG) neurons were measured in the isolated brainstem-spinal cord from newborn rats during bath application of the opioid receptor agonists fentanyl or [D-Ala2, D-Leu5]-enkephalin (Ala-Leu-Enk) and of the prostaglandin E1 (PGE1). 2. PGE1 (0.1-3 microM) and fentanyl or Ala-Leu-Enk (1-50 microM) produced depression and, at higher doses, block of inspiratory nerve activity and respiration-related postsynaptic potentials. This apnoea was associated with hyperpolarization and Rm fall in 25% of thirty-two VRG neurons tested, whereas resting Vm and Rm were not changed in the other cells. 3. The selective mu- and delta-receptor blockers naloxonazine (10-20 microM) and naltrindole (50-100 microM) antagonized the effects of 5 microM fentanyl and 50 microM Ala-Leu-Enk, respectively. 4. Opioid- and PGE1-evoked respiratory depression was reversed upon elevation of endogenous cAMP levels by stimulating adenylyl cyclase with 100 microM forskolin, activating dopamine D1 receptors with 50-100 microM 6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2, 3,4,5-tetrahydro-1H-3-benzazepine (6-chloro-APB) or preventing cAMP breakdown with 50-100 microM isobutylmethylxanthine. 5. The results indicate that opioid- or prostaglandin-induced respiratory depression is due to a fall in cAMP levels in cells responsible for generation of rhythm or providing a tonic drive to the respiratory network. 6. We suggest that elevation of cAMP levels is an effective antidote in neonates against such forms of respiratory depression.