The liver production of the insulin-like growth factor-I (IGF-I) is a key factor in the endocrine control of body growth by a growth hormone. As pejerrey Odontesthes bonariensis has been reported as a fish with low growth rates in captivity, basic research on this respect is needed in order to understand it. In this context, the pejerrey IGF-I cDNA was cloned and its hepatic expression was examined in fish after recombinant pejerrey growth hormone (pjGHr) administration. The full length of IGF-I transcript showed a high sequence similarity to other teleost sequences. The tissue distribution analysis by reverse transcriptase polymerase chain reaction in adult fish revealed that IGF-I expressed ubiquitously with the highest mRNA levels in the liver, posterior intestine and brain. No alternative IGF-I mRNA was found in the liver, as it was reported for other teleosts. IGF-I transcript was measured in the liver after pjGHr in vivo stimulation by means of quantitative real-time polymerase chain reaction assays. A dose-dependent response of IGF-I mRNA was observed after pjGHr administration, and reached a six-fold IGF-I maximum increase over control group when 2.5 microg pjGH/g-body weight (bw) was injected. Temporal analysis of hepatic IGF-I mRNA level showed that administration of a single dose of pjGHr into juvenile pejerrey resulted in a significant increase (P <0.02) 9 hours post-injection (hpi). These results add novel information on the nucleotide sequence of IGF-I in Atheriniformes and demonstrate that pjGHr could promote a dramatic response in liver, increasing the IGF-I mRNA level.