An electromagnetic field (EMF) of extremely low frequency may affect physiological processes in mammals. The aim of the present study was to determine the effect of an EMF on the synthesis and secretion of oestradiol-17β (E 2 ) in the porcine uterus. Endometrial and myometrial slices were harvested on days 12-13 of the oestrous cycle and exposed in vitro to an EMF (50 and 120 Hz, 8 mT) for 2 and 4 h in the presence or absence of progesterone (P 4 ). Subsequently, the incubation media were used to determine the concentration of E 2 with RIA. Tissues fragments were used to study the expression of CYP19A3 mRNA using Real-Time PCR and the abundance of P450 aromatase using Western Blotting. The 50-Hz EMF increased E 2 release from the endometrium and the myometrium at both time points of in vitro incubation. A 120-Hz EMF decreased the endometrial secretion of E 2 after 2 h of incubation and did not affect E 2 secretion after 4 h. In the myometrium, the 120-Hz EMF increased E 2 secretion after 4 h of incubation. In P 4 -treated uterine fragments, no significant EMF exposition-related changes were observed. Only myometrial fragments incubated in the presence of P 4 at 120-Hz EMF (4 h) released higher amounts of E 2 due to EMF treatment. The 50-Hz EMF exposure did not change the CYP19A3 mRNA expression in endometrial fragments incubated in the presence or absence of P 4 . In myometrial fragments, the highest CYP19A3 mRNA expression was observed in fragments not exposed to the 50-Hz EMF and P 4 -treated tissues compared to that in fragments exposed to 50 Hz EMF and incubated with or without P 4 and control (no EMF and no P 4 ) fragments. The EMF at 120 Hz decreased basal endometrial CYP19A3 mRNA expression and did not change the expression in the P 4 -treated endometrium. In the myometrium, the EMF at 120 Hz increased CYP19A3 mRNA expression in slices incubated without P 4 and had no effect in the presence of P 4 . The EMF exposure (50 and 120 Hz) did not affect P450 aromatase abundance in either the endometrium or the myometrium. In conclusion, the EMF induces changes in the synthesis and release of E 2 in uterine tissues harvested during days 12-13 of the oestrous cycle. These changes are related to the EMF frequency used, the time of the exposition and the presence of P 4 . We suspect that this observed phenomenon might lead to changes in the intrauterine milieu of oestrogen, which is crucial for the proper activity of uterine tissues during the mid-luteal phase of the oestrous cycle., (Copyright © 2017. Published by Elsevier Inc.)