Achondroplasia was induced in chicken embryos by in ovo application of 0.6 mg/egg thallium sulfate. The critical (sensitive) period for production of achondroplasia began on day 5 of incubation and ended at the start of HH stage 35 (8.5 days). The end of the critical period was accurately timed and found to be 205–207 hours of incubation and to coincide with a 66% decrease in growth rate of the embryos. Treatment resulted in reduced tibial growth one day later, tibial angulation two clays later, and chondrocytic necrosis four days later. The last was therefore not the cause of the angulation. Tibias were taken from thallium-treated and control donor embryos of various ages and grafted to the chorioallantoic membranes of treated and control host embryos of various ages during and outside the critical period and achondroplastic changes induced in grafted tibias exposed to thallium while on the chorioallantoic membrane. The critical period was extended into day 10 of incubation in such grafted tibias. Tibias maintained for seven days in organ culture were achondroplastic if pretreated with thallium at seven or eight days of incubation but not at ten days. Exposure of as little as 0.5 hour was sufficient to elicit micromelia when the tibias were grafted or organ cultured. Thallium therefore rapidly binds to skeletal tissues during a critical period of embryonic development but this critical period may be extended when tibias are removed from the embryo.