Superovulation is induced in the female rodent with a priming dose of PMS followed by an ovulating dose of HCG. At proper time intervals thereafter, follicular or ductal ova can be obtained in large numbers, freed of coronal cells while in a 0.7 % sodium citrate solution and fixed in 50 % acetic acid. Freezing with dry ice and methanol permits removal of the cover slip after mild squashing and the dried preparation can then be hydrolysed with in HCL and stained with Giemsa stain. Preparations of good quality which permit examination of number and morphology of the chromosomes of ova in first or second meiotic metaphase can thus be obtained.