The characterization of (pro)insulin as an early embryonic growth factor requires demonstration of its expression and cellular effects in vivo .B yin situ hybridization, we found widespread preproinsulin transcripts in the chick embryo throughout gastrulation and neuru- lation, before the beginning of preproinsulin-like growth factor I ex- pression and pancreatic organogenesis. To analyze the prepancreatic (pro)insulin effect on apoptotic cell death, we treated embryos with antisense oligodeoxynucleotides in ovo and in vitro. The specific effect of two preproinsulin messenger RNA (mRNA) antisense oligode- oxynucleotides was confirmed by the decrease in a biosynthetically labeled protein immunoprecipitated with antiinsulin Igs. Insulin re- ceptor mRNA antisense oligodeoxynucleotide applied in ovo increased by 2.7-fold the level of apoptosis in the 1.5-day embryo (neurulation) compared with that in its random sequence control. In a whole embryo culture, apoptosis increased by 25-35% with the addition of prepro- insulin or insulin receptor mRNAs antisense oligodeoxynucleotides, respectively, whereas it decreased by 64% after 10 h in the presence of 1028 M chicken insulin. Exogenous insulin also rescued the death induced by preproinsulin antisense oligonucleotides. These findings provide evidence for an autocrine/paracrine role of preproinsulin gene products acting through the insulin receptor in the control of cell survival/death during early embryonic development. (Endocrinology 138: 3967-3975, 1997)