Insulin-like growth factor-I (IGF-I) is essential for cell growth, differentiation and postnatal development. A null mutation in igf-1 causes intrauterine growth retardation and perinatal lethality. The present study was designed to test the lower limit of igf-1 gene dosage that ensures survival and postnatal growth by using the Cre/loxP system. Mice with variable reductions in IGF-I levels were generated by crossing EIIa-cre transgenic mice and mice with loxP-flanked igf-1 locus (igf-1/flox). EIIa-cre mice express bacteriophage P1 Cre (causes recombination) recombinase under the adenovirus promoter EIIa, during early embryonic development before implantation, and cause genomic recombination of the igf-1/flox locus. Mice with the most extensive recombination die immediately after birth, while the survivors have significant growth retardation in proportion to the reduction in their igf-1 gene. Interestingly, this gene dosage effect on body weight was not very significant before weaning. However, when the young animals were weaned at 3 weeks, the igf-1 gene dosage was the only independent predictor of the weight gain between 3 and 6 weeks among the parameters tested. Although growth retarded, mice with Cre-induced partial igf-1 deficiency were fertile and gave birth to null mice. Thus Cre-induced genomic recombination using the EIIa promoter occurs during development and creates distinct phenotypes compared with the conventional null mutation. This variability allows for postnatal survival and will enable one to begin to explore the role of the endocrine vs. paracrine effects of IGF-I.