The expression of the insulin like growth factor (IGF) II gene has been examined in the developing equine fetus. It was found that IGF II transcripts were present in abundant quantities in third trimester embryonic and extraembryonic tissues as for example the placenta. The expression of the IGF II gene was high in the fetal liver where two prominent transcripts — 4.6 and 4.1 — kB were produced. However, these transcripts could not be traced in the adult liver. Instead we found two different transcripts with the sizes of 4.0 and 2.9 kB in the adult liver. These findings taken together with the demonstration of heterologous IGF II cDNA hybridizing to equine DNA-digests suggests that the IGF II gene is under developmental control in the horse, with the possible existence of different promoters.