Cytochrome P450 enzymes generate epoxyeicosatrienoic (EETs) and hydroxyeicosatetraenoic acids (HETEs) from arachidonic acid and act to decrease or increase smooth muscle contractility, respectively. As such, they may be important factors in the control and/or initiation of labour and differential expression of EETs or HETEs generating enzymes may therefore be predicted at key stages of pregnancy. Myometrial biopsies, gathered from pregnant women at elective or emergency caesarean section (term, 38+ weeks) after full informed written consent were utilised in these studies. Samples (1.1 ± 0.5g) of uterine smooth muscle were washed and snap frozen in liquid nitrogen for Western blotting or enzymatically dispersed and cultured for immunocytochemistry within 24 hours of collection. For Western blotting, samples were incubated with antibodies to either CYP450 2C9/19 (EETs generating) or CYP450 4A (HETEs generating) enzymes following SDS-PAGE. The same antibodies were used for immunocytochemistry. Data is shown as mean ± S.E mean, where used a t-test was done to determine significance at the level P0.05) between labouring (n=6) (46.4 ± 0.1 kDa vs 45.5 ± 0.2 kDa) and non-labouring myometrial samples (n=7), nor was there any significant difference in band density (26.4 ± 1 units vs 29.4 ± 1.8 units respectively, t-test P=0.19). Comparison of labouring (n=6) and non-labouring (n=7) samples for CYP 2C9/19 showed similar molecular weights at 44.4 ± 0.3 vs 45 ± 0.7 kDa (t-test P>0.05) respectively and no significant differences in band density 22 ± 8.2 units vs 33.7 ± 15 units respectively, (t-test P>0.09). Beta-actin loading controls for all groups had similar molecular weights (42 kDa) and no significant differences in band density (t-test, P>0.1). Strong immunoreactivity to CYP450 4A and feint reactivity to 2C9/19 was also found in cultured non-labouring smooth muscle cells. In conclusion, both the HETE (4A) and EET generating (2C9/19) P450 enzymes were detected in pregnant myometrium. Although no significant differences in expression of CYP 2C9/19 and CYP 4A were identified using Western blotting in non-labouring and labouring myometria, it may be argued that labouring myometrium is shifted more towards a HETE generating phenotype than that of EETs, with resultant HETE activity contributing towards successful labour. Additional studies are underway to validate these observations.