We have previously shown that mRNA for caeruloplasmin, the serum copper-binding protein, is not expressed in guinea-pig liver prior to birth and expression increases rapidly following parturition. To further study the regulation of caeruloplasmin in neonatal animals we have used 3-day preterm guinea pigs, delivered by caesarean section, to investigate mRNA expression in liver and lung. Within 12 h of premature birth, hepatic caeruloplasmin mRNA levels are significantly increased and remain elevated by 72 h. This induction of expression is accompanied by an increase in circulating levels of holoprotein. Even greater induction of caeruloplasmin mRNA was observed in premature animals maintained in 95% oxygen for 72 h, confirming an acute-phase response in prematurity. Studies of lung RNA showed that caeruloplasmin mRNA is expressed throughout development, with highest levels observed in adult lung. In the premature animals levels were significantly elevated 12 h after delivery, but then fell by 72 h to below those seen in normal term lung. Hyperoxia did not influence the pulmonary mRNA levels.