The rho family of small G-proteins are postulated to be involved in the generation of uterine contractions associated with successful parturition. For instance, pharmacological inhibition of rho-associated kinase (ROK), a downstream effector of rhoA, reduces agonist-induced Ca²⁺-sensitisation and contractility of myometrial strips in vitro (Oh et al. 2003; Moran et al. 2002). Recent reports suggest that constitutively active members of the rho family, termed rnd1 or rnd3, may impede Ca²⁺-sensitisation during pregnancy and that rnd3 protein, and rnd1 mRNA, are up-regulated at mid- to late gestations in rabbit and rat myometria respectively (Kim et al. 2003; Cario-Toumaniantz et al. 2003). However, the expressions of rnd proteins in human myometria have yet to be determined. We, therefore, examined the content of rnd3 protein in myometrial biopsies isolated, following written informed consent, from women undergoing hysterectomy (non-pregnant; NP) or Caesarean section at term (37-39 weeks) not in labour (PNL), in spontaneous labour (SL) or in oxytocin-stimulated labour (OT). Such biopsies were processed for SDS-PAGE/Western blotting with anti-rnd3 monoclonal antibody (1:500; Upstate) and anti-α-actin monoclonal antbody (1:100,000; Sigma) and ECL signals quantified by densitometry. Myometrial α-actin content remained invariant during late pregnancy and labour compared to NP. In contrast, rnd3 expression, when correlated to α-actin, increased significantly near to term: the rnd3:α-actin ratio for myometria from PNL women was 1.89±0.35-fold that of corresponding values from NP tissues (P〈0.05, unpaired t-test; mean ± S.E.M., n=4). Rnd3 levels were elevated further in myometria from labouring women: rnd3:α-actin ratios were raised in tissues from SL and OT to 2.07±0.09-fold and 1.93±0.30-fold, respectively, of myometria from PNL women (n=4). This data suggests that late pregnancy, and labour onset, is associated with elevated myometrial expression of rnd3. This appears counter-intuitive unless rnd3 has a functional role(s) that (i) is in addition to that suggested previously of limiting Ca²⁺-sensitisation of contraction and/or (ii) is different in human myometrium than so far proposed for rnd proteins in rat (Kim et al. 2003) or rabbit (Cario-Toumaniantz et al. 2003) myometrium.