Recently, a rationalization of strains used in our laboratories has led to the adoption of the Han Wistar rat as the standard strain for repeat dose and reproductive toxicity studies. Earlier work on developing an extended whole embryo culture model in which AHA (Allen and Hanbury Albino) rats were grown from day 10 to day 14 of pregnancy was therefore repeated in the Han Wistar strain. Differences were seen in rate of embryonic growth in vivo for the two strains of rat, particularly in terms of crown-rump length and protein content. No such differences were seen in rate of morphological development. Extending the period of whole embryo culture up to day 14 of pregnancy using a novel protocol further illustrated these differences in growth characteristics. Results showed marked differences in growth, but not in general morphological development, when a standard culture technique was used. These findings indicate that accepted standard culture regimens are unsuitable for some strains of rat, and that the development of strain-specific methodology must be considered when such models are to be used for mechanistic studies.