Serum GH, IGF-I, IGF-II and IGFBP-2 concentrations were determined by radioimmunoassay in swine of genetic lines which were selected for high (obese) and low (lean) backfat. Blood samples were collected at birth, before and after nursing, at 1 and 3 days of age and at weekly or fortnightly intervals until 30 weeks of age. Overall, GH, IGF-I, IGF-II and IGFBP-2 were highest at birth and declined during the first week of postnatal life. An age-by-line interaction was apparent for GH and IGF-I during the early neonatal period with levels being higher in the lean line than the obese line at 1 day of age and similar at 1 week of age. At 3 to 5 weeks of age there was an elevation in GH which was greater in lean than obese pigs. IGFBP-2 concentration patterns were characterized by a nadir at 5 to 7 weeks of age and a decline from an apex at 8 weeks of age in both lines. IGF-II declined steadily from birth until about 10 weeks of age. A subsequent increase in IGF-II was then observed between 12 and 22 weeks, which was greater in the obese line and in male pigs but not apparent in lean females. At birth, pigs which had not nursed had higher GH and IGFBP-2 and lower IGF-I and IGF-II concentrations. The effect of nursing on IGF-I was significantly influenced by line.(ABSTRACT TRUNCATED AT 250 WORDS)