Under normal circumstances, fertility and embryotoxicity studies are run separately according to the ICH S5(R2) guideline for the detection of toxicity to reproduction of medicinal products (1). However, the flexible approach of the S5(R2) guideline also allows the reproduction stages covered in the fertility and embryo-fetal development studies (stages A to D) to be combined into a single study design. The administration period covers the pre-mating and gestation phases through to closure of the hard palate. The principal advantages of the combined study include reductions in the number of animals required and cost. Although the rat is the routine species of choice, the mouse may also be used.