The quantitative relationship between developmentally toxic exposure levels and adult toxic exposure levels has been used as an index of developmental hazard and has figured prominently in discussions of legal regulation of developmentally toxic agents. The authors explored some quantitative aspects of the A/D ratio and of the concept of developmental hazard by simulating 661,500 mammalian developmental toxicity assays on 441 hypothetical compounds. In study simulations, A/D often varied substantially among replicate assays: the median ratio of the upper and lower limits of the distribution of A/D values that include about 95% of the observed A/D values is 16. In addition, A/D did a poor job of predicting the relative developmental and adult responses at dosages lower than those used to calculate the index: among simulated compounds with A/Ds of about 1.0, the developmental response at 1/100th of the NOAEL ranged from about 0.1% to 20,000% of the adult response. It seems likely that no single index can quantify 'developmental hazard,' as defined by relative adult and developmental susceptibility, and more effort needs to be expended in refining the concept if it is to be useful for hazard assessment.