Severe insulin resistance is a defining attribute of gestational diabetes mellitus (GDM). It is postulated that alterations in the insulin-signalling pathway and subsequent glucose disposal are the underlying cause of insulin resistance in patients with GDM. The purpose of this study was to profile the insulin-signalling pathway and intermediates in insulin-sensitive tissues. Subcutaneous adipose tissue and skeletal muscle were collected from normal glucose-tolerant (NGT) and insulin-controlled GDM in both non-obese and obese cohorts (n=6-8 per subgroup). Expression studies of the insulin-signalling pathway were performed using western blotting and quantitative reverse transcription-PCR. This study demonstrated altered mRNA expression of insulin receptor substrate (IRS)-1, IRS-2, glucose transporter (GLUT)-1, GLUT-4 and glycogen synthase kinase (GSK)-3 isoforms genes in adipose tissue in GDM women in comparison to NGT pregnant controls. In skeletal muscle, insulin-controlled GDM was associated with decreased IRS-1, phosphatidylinositol-3-kinase (PI3-K) p85alpha, GLUT-1 and -4, GSK-3 isoforms and phosphoinositide-dependent kinase-1. Both adipose tissue and skeletal muscle from women with GDM displayed decreased IRS-1 and GLUT-4 and increased PI3-K p85alpha protein expression. Both skeletal muscle and adipose tissue from obese women demonstrated lower GLUT-1 and -4 mRNA expression and diminished GLUT-4 protein expression in skeletal muscle only. Collectively, our results suggest that diabetes and obesity during pregnancy cause defects in insulin-signalling transduction in adipose tissue and skeletal muscle and may be the underlying cause of GDM.