Insulin resistance is a hallmark of late pregnancy both in human and rat. Adipose tissue is one of the tissues that most actively contributes to this reduced insulin sensitivity. The aim of the present study was to characterize the molecular mechanisms of insulin resistance in adipose tissue at late pregnancy. To this end, we analyzed the insulin signaling cascade in lumbar adipose tissue of non-pregnant and pregnant (day 20) rats both under basal and insulin-stimulated conditions. We found that the levels of relevant signaling proteins, such as IR, IRS-1, PI3K, PDK-1, ERK1/2 and PTEN did not change at late pregnancy. However, insulin-stimulated tyrosine-phosphorylation of both IR and IRS-1 were significantly decreased, coincident with decreased IRS-1/p85 association and impaired phosphorylation of Akt/PKB and ERK1/2. This impaired activation of IRS-1 occurred together with an increase of IRS-1 phosphorylation at serine 307 and a decrease in adiponectin levels. To corroborate the role of IRS-1 in adipose tissue insulin resistance during pregnancy, we treated pregnant rats with the anti-diabetic drug englitazone. Englitazone improved glucose tolerance and this pharmacological reversion of insulin resistance was paralleled by an increase of adiponectin levels in adipose tissue as well as by a reduction of IRS-1 serine-phosphorylation. Furthermore, the impaired insulin-stimulated tyrosine-phosphorylation of IRS-1 in adipose tissue of pregnant animals could be restored ex vivo by treating isolated adipocytes with adiponectin. Together, our findings support a role for adiponectin and serine phosphorylation of IRS-1 in the modulation of insulin resistance in adipose tissue at late pregnancy.