RHO GTPases are small monomeric proteins that play a key role in the regulation of the actin cytoskeleton. Transfection of activated RHOA and RHOB GTPases into quiescent fibroblasts induces the formation of actin stress fibres and adhesion complexes [1]. Although RHOB GTPase has over 89% sequence homology to RHOA, it is differentially localised, regulated and prenylated. RHOB is localised to endosomes where it regulates receptor-mediated vesicle trafficking and actin filament assembly through the RHO effector protein mammalian Diaphanous-related formin (DRF) Dia1 [2]. Activated RHOB promotes the formation of thin actin filaments and inhibits vesicle motility by associating endosomes with subcortical F actin filaments [3]. We aim to quantify the presence and expression of RHOA and RHOB GTPases in human placenta. We also aim to determine the expression of RHOB in other intrauterine tissues as it could be an important confounding factor on previous reports. Whole tissue extracts of myometrium and placenta from five individual non-pregnant and pregnant samples were prepared by mechanical homogenisation and protein expression was determined using immunoblotting and densitometry. mRNA expression of RHOA and RHOB in placental samples relative to pregnant myometrial samples from the same patient samples was determined using quantitative RT-PCR. Immunohistochemical localisation of paraffin-embedded placental tissue sections from five individual patient samples was demonstrated using antibodies against RHOA and RHOB GTPases. RHOA and RHOB GTPase mRNA were both expressed in the placental tissue samples. RHO GTPase protein expression was high in non-pregnant and pregnant myometrium, decidua basalis and decidua parietalis, fetal blood and placental tissues. Immunohistochemical analysis revealed that RHOB GTPase has a cytoplasmic distribution in myometrial cells, with staining localised to the perinuclear region in endometrial cells. In human placenta, trophoblast and decidua cells stained positive for RHOB GTPase. To our knowledge, this is the first time RHOB GTPase mRNA and protein expression has been described in human myometrial and placental tissues. We have also demonstrated that both RHOA and RHOB GTPase are expressed in myometrial, decidual and placental tissues samples. This is an important confounding factor on previously reports of RHOB protein expression in human myometrium. We aim to further explore RHOB GTPase function in placenta by identifying possible effectors using immunoprecipitation and immunoblotting.