Abstract. IGF I was determined by a radioimmunoassay and IGF II by a radioreceptorassay in 20 Göttinger miniature (mini)-pigs and 13 domestic pigs of different weight and age. Immunoreactive IGF I serum levels of mini-pigs were similar to those of domestic pigs in corresponding age-classes (150–250 and 100–270 μg/l, respectively). No differences were detectable between receptor-reactive IGF II serum levels in mini-pigs (150–200 μg/l) and domestic pigs (110–270 μg/l) nor did the biological insulin-like activites (measured in the rat fat cell assay) differ in mini- and domestic pigs (81–100 and 71–98 mU insulin/l, respectively). IGF I and IGF II decreased drastically after hypophysectomy in one of the mini-pigs. Intravenous bolus injections of 30 μg/kg of recombinant human IGF I in 4 mini-pigs caused a similar degree of hypoglycemia (nadir of blood glucose 1.33 ± 0.61 mmol/l) as 0.15 IU insulin/kg, followed by a sharp growth hormone peak. We conclude that the marked difference between mini- and domestic pigs regarding body size is unrelated to serum levels of IGF I and II, a lack of response of tissues to IGF I or a reduced growth hormone secretory capacity in the mini-pig.