Rats were maintained in chambers and breathed air (control, n=8) or an atmosphere containing 10% oxygen (hypoxic, n=10) for 35 days. On completion of the experiment the hypoxic animals weighed less than the controls (hypoxic, 290 ± 11.7g; control, 339 ± 19.2g; means ± S.E.M., p<0.05). No differences in the left ventricular weights were found between groups but the right ventricular weights were greater in the hypoxic rats (hypoxic, 0.39 ± 0.02g; control, 0.27 ± 0.08g; p<0.01). The amount of mRNA for IGF-I in the ventricles was quantified by Northern blot analysis. There was no difference between groups in IGF-I mRNA levels in the left ventricles (hypoxic, 1.07 ± 0.41 absorbance units (AU); control, 0.73 ± 0.33 AU). In the right ventricles, IGF-I mRNA was greater in hypoxic than in control rats (hypoxic, 2.37 ± 0.75 AU; control, 0.64 ± 0.11 AU; p<0.05). This study demonstrates that expression of IGF-I mRNA is increased in the hypertrophied right ventricle of hypoxic rats; IGF-I may play a central role in the initiation and maintenance of this process.