We have investigated the availability of circulating maternal insulin to the postimplantation rat conceptus during early organogenesis, before the insulin-impermeable allantoic placeta is established. Virtual distribution equilibrium for "tracer" quantities of 125I-porcine insulin was achieved by continuous infusions into pregnant rats on day 11 of gestation (day 10.3 of embryo development). During continuing infusion, following 210 min of iodoinsulin delivery, intact conceptuses (embryo, amnion, and yolk sac), and portions of adjacent decidua, liver, and spleen were excised, rinsed, and frozen in liquid N2 within 2 min. Subsequent analysis by trichloroacetic acid (TCA) precipitation or immunoprecipitation revealed the presence of intact iodoinsulin in all tissues. Both analytical approaches disclosed the same pattern of iodoinsulin distribution, i.e., liver and spleen greater than decidua and conceptus. Assayable iodoinsulin could not be demonstrated in embryos excised from the intact conceptuses; however the embryo observations were limited by the amount of accessible tissue and the requisite preparative delay. Our studies indicate that during early organogenesis, before the establishment of the allantoic placenta, maternal insulin has access to at least the outer portion of the postimplantation rat conceptus, including yolk sac, where insulin-specific receptors are known to occur.