The postnatal time courses in preterm infants (30.0 weeks mean gestational age) of serum copper and ceruloplasmin (Cp) are described and compared. Serum copper was measured by nonflame atomic absorption spectrophotometry and serum Cp was measured by both immunodiffusion (Cpi) and enzyme assay (Cpe). With the exception of Cpe between 7 and 11 weeks of age, correlations among the three laboratory parameters were excellent. Following an early rise in all values, a plateau was seen in copper and Cpi between 5 and 11 weeks; Cpe showed a discordant fall at 7 to 11 weeks of age. After 11 weeks a secondary and definitive rise in all values was seen. Examination of individual time courses indicates the existence of a subgroup of infants with falling values between 5 and 11 weeks of age. Preterm infants had significantly higher serum copper (0.53 +/−0.05 micrograms/ml) at a mean postnatal age of 10 weeks (39-41 weeks postconception) than did full-term infants (0.44 +/−0.04 microns/ml) of similar postconceptional age. Low serum copper values were correlated with parenteral nutrition deficient in copper, with increased growth rates, and with twin pregnancies. It is concluded that extrauterine life in preterm infants is associated with precocious rises in serum copper and Cp values. The subgroup of infants with falling values may represent subclinically copper-deficient infants. This subgroup may bias the results of grouped data downwards. A role for nutritional factors in the delayed postnatal rise of serum copper and Cp values in preterm infants cannot be excluded on the basis of current knowledge.