Zearalenone (ZEA) is an undesirable substance in feed materials and feed of plant origin. It is an example of the micotoxin that causes disturbances in the functioning of the reproductive system. The wide range of plant compounds in pet food means that ZEA may frequently have a negative effect on pet reproduction. An assessment of the influence of ZEA on the granulosa and theca cells of the ovarian follicle in bitches in vitro was carried out. The co-culture of the ovarian follicles was incubated for 72 hours with the addition of 12.5 ng/ml and 25.0 ng/ml of ZEA. Numerous vacuoles in the cytoplasm of the granulosa cells were noted in the culture with the addition of 25.0 ng/ml of ZEA. Preliminary investigations suggest negative effect of ZEA on the granulosa cells in bitches in vitro.