The insulin-like growth factor (IGF) system is regulated by various stimuli, including hormones, growth factors and nutritional status. We examined the effects of vitamin A on components of the IGF system in Japanese quail. Male quail (1 d old) fed a vitamin A-deficient diet for 14 or 21 d developed vitamin A deficiency, as confirmed by a depletion of serum retinol and hepatic retinyl palmitate. Consuming the vitamin A-deficient diet for 14 d did not affect growth rate, but decreased the serum IGF-I concentrations by 22% compared with the control group. The decreased serum IGF-I levels were accompanied by 21-52% lower levels of IGF-I mRNA in the testis, lung, liver and heart, whereas IGF-I receptor (IGF-IR) and insulin receptor (IR) gene expressions were unaffected in these tissues. Continuous feeding of the vitamin A-deficient diet for 21 d retarded growth and further decreased the levels of serum IGF-I and tissue IGF-I mRNA. Serum IGF-I levels were reduced by approximately 50%; IGF-I mRNA levels were > 90% lower in the liver and lung and approximately 60% lower in the heart and testis. In contrast, levels of the IGF-IR and IR mRNAs were approximately 100% greater in some tissues examined. When vitamin A-deficient quail received a single injection of retinol or retinoic acid (0.1 mg/bird), tissue IGF-I, IGF-IR and IR gene expressions did not change after 4 h. These results suggest a possible physiologic role of the IGF system in mediating vitamin A-supported growth of Japanese quail.