The variation in the quantity of circulating chorionic gonadotrophin (CG) and its follicle-stimulating hormone (FSH) and luteinizing hormone (LH) activity in rodent bioassay systems was investigated. A portion of the variability in total CG could be attributed to the stallion that sired the pregnancy and it was possible to select sires and mares to increase CG production. It was further demonstrated that FSH activity per unit of CG was greater at Days 71 and 104 of gestation than at Day 39. LH activity per unit of CG varied with the sire, but no effect of day of gestation could be shown. It was demonstrated that removal of sialic acid increased LH bioactivity and it is proposed that variation in biological activity between animals and during gestation may be a function of differences in carbohydrate content of CG.