The endogenous cannabinoid system plays an important role in regulating the immune system. Modulation of endogenous cannabinoids represents an attractive alternative for the treatment of inflammatory disorders. This study investigated the effects of URB597, a selective inhibitor of fatty acid amide hydrolase (FAAH), the enzyme catalysing degradation of the endogenous cannabinoid anandamide, and AM404, an inhibitor of anandamide transport, on lipopolysaccharide (LPS)-induced increases in plasma cytokine levels in rats. Both URB597 and AM404 potentiated the LPS-induced increase in plasma tumour necrosis factor-alpha (TNF-alpha) levels. The peroxisome proliferator-activated receptor gamma (PPAR gamma) antagonist, GW9662, attenuated the AM404-induced augmentation of TNF-alpha levels. Furthermore, the selective cannabinoid CB(1) and CB(2) receptor antagonists, AM251 and AM630 respectively, and the transient receptor potential vanilloid receptor-1 (TRPV1) antagonist, SB366791, reduced LPS-induced TNF-alpha plasma levels both alone and in combination with AM404. In contrast, AM404 inhibited LPS-induced increases in circulating interleukin-1 beta (IL-1 beta) and IL-6. AM251 attenuated the immunosuppressive effect of AM404 on IL-1 beta. None of the antagonists altered the effect of AM404 on LPS-induced IL-6. Moreover, AM251, AM630 and SB366791, administered alone, inhibited LPS-induced increases in plasma IL-1 beta and IL-6 levels. In conclusion, inhibition of endocannabinoid degradation or transport in vivo potentiates LPS-induced increases in circulating TNF-alpha levels, an effect which may be mediated by PPAR gamma and is also reduced by pharmacological blockade of CB(1), CB(2) and TRPV1. The immunosuppressive effect of AM404 on IL-1 beta levels is mediated by the cannabinoid CB(1) receptor. Improved understanding of endocannabinoid-mediated regulation of immune function has fundamental physiological and potential therapeutic significance.