Serum PRL levels were followed serially in full term (FT; 39-41 weeks) and preterm (PT; 30-32 weeks) infants, from birth to 12 and 20 postnatal weeks, respectively. Values were higher in FT infants than in PT infants on day 1 after birth (267 +/- 20 vs. 156 +/- 8 ng/ml) but were similar in both by the age of 2-4 weeks (69 +/- 8 vs. 69 +/- 6 ng/ml). Between the ages of 4-12 weeks, the serum PRL in FT infants fell to near adult levels (24 +/- 2 ng/ml), but this fall was seen much later in PT infants, between 12-20 weeks postnatally (23 +/- 2 ng/ml). When values in FT and PT infants were compared at parallel postmenstrual ages in contradistinction to postnatal ages, a similar course of PRL was discernable in both groups. These data may provide indirect evidence for the establishment and maturation of inhibition of PRL secretion (i.e. PRL-inhibitory factor production) postnatally, between 44-52 weeks postmenstrually.