Insulin and IGF-I binding to semi-purified red muscle receptors was characterized in brown trout, Salmo trutta and the common carp. Cyprinus carpio. The yield of glycoprotein obtained after semipurification of receptors with WGA-agarose affinity chromatography in microgram g-1 initial tissue was 210.6 +/- 21 micrograms g-1 in trout and 108.5 +/- 2.5 micrograms g-1 in carp. IGF-I specific binding (4.72 +/- 0.64%/10 micrograms glycoprotein) was 4-5-times higher than insulin binding (1.04 +/- 0.12%/10 micrograms glycoprotein) in trout red muscle. This difference in binding was due to a higher number and a greater affinity of the IGF-I (Kd, 0.21 +/- 0.03 nM) compared with the insulin (Kd, 0.67 +/- 0.06 nM) receptors in this tissue. Carp red muscle IGF-I binding (9.14 +/- 0.55%/10 micrograms glycoprotein) surpassed insulin binding (2.59 +/- 0.094%/10 micrograms glycoprotein) mainly because of a greater affinity of the IGF-I (Kd, 0.092 +/- 0.027 nM) compared with the insulin (Kd, 0.1515 +/- 0.0285 nM) receptor. IGF-I and insulin binding in carp red muscle were higher than in trout, as a consequence of a higher affinity of carp red muscle receptors. Arginine injection provoked acute hyperinsulinemia in both trout (23.3 +/- 1.01 ng ml-1) and carp (24.3 +/- 1.34 ng ml-1. Specific binding of insulin and IGF-I to the red muscle decreased 4 h after injection. In trout, a decrease of insulin and IGF-I binding of 47.0% and 63.3%, respectively was observed compared with controls, in carp, these values were 44.0% and 45.0%. The number of insulin and IGF-I receptors decreased (42-55%) but affinities did not change suggesting that receptor down-regulation is a consequence of high insulin levels.