Breathing response to 12% and 6% O2 in N2 (at isocapnia) was measured in anaesthetized piglets, 1-5 and 19-25 days old, before and after 3 mg kg-1 i.v. naltrexone. The degree of interaction between the anaesthetic and naltrexone was assessed. At the end of each hypoxic trial, arterial blood was sampled for measurements of pH and gas tensions, (Met)enkephalin-Arg6-Phe7, adenosine, noradrenaline and adrenalin. Whereas respiration in older animals was stimulated by hypoxia, young piglets had a biphasic response with a pronounced ventilatory decrease in response to severe hypoxia (6% O2/N2). In young animals there was a greater ventilatory response with naltrexone than without the drug, and the biphasic hypoxic response was ameliorated or reversed by naltrexone. Levels of adrenalin increased and those of encephalin, adenosine and noradrenaline tended to increase during hypoxia in the younger age group. Levels of adenosine showed significant increase when data from both age groups and levels of hypoxia were pooled. Combined with previously reported physiological evidence regarding adenosine in hypoxic depression, we conclude that the present results are compatible with a role of opioid peptides and adenosine in the early postnatal response to hypoxia.