Nitric oxide synthase (NOS) activity was detected in soluble and membranous fractions of adipose tissue homogenates of control rats. After LPS-treatment, this activity was (i) markedly increased (about 10-fold) in both fractions, (ii) unaltered after dexamethasone pretreatment, (iii) partly calcium-calmodulin sensitive, and (iv) almost entirely accounted by the NOS activity found in isolated adipocytes. In adipose tissue homogenates from control rats, Western blot analysis demonstrated the presence of the endothelial (eNOS) isoform in the membranous fraction of control rats and of the inducible (iNOS) isoform in the soluble and membranous fractions. After LPS treatment, the amount of immunoreactive iNOS protein was dramatically increased, suggesting that adipose tissue is an important site of NO production during the endotoxic shock.