--- - |2+ Abstract Aim Whereas some patients have important changes in body core temperature (Tb) during systemic inflammation, others maintain a normal Tb, which is intrinsically associated to immune paralysis. One classical model to study immune paralysis is the use of repeated administration of lipopolysaccharide (LPS), the so‐called endotoxin tolerance. However, the neuroimmune mechanisms of endotoxin tolerance remain poorly understood. Hydrogen sulphide (H2S) is a gaseous neuromodulator produced in the brain by the enzyme cystathionine β‐synthase (CBS). The present study assessed whether endotoxin tolerance is modulated by hypothalamic H2S. Methods Rats with central cannulas (drug microinjection) and intraperitoneal datalogger (temperature record) received a low‐dose of lipopolysaccharide (LPS; 100 µg kg−1) daily for four consecutive days. Hypothalamic CBS expression and H2S production rate were assessed, together with febrigenic signalling. Tolerant rats received an inhibitor of H2S synthesis (AOA, 100 pmol 1 µL−1 icv) or its vehicle in the last day. Results Antero‐ventral preoptic area of the hypothalamus (AVPO) H2S production rate and CBS expression were increased in endotoxin‐tolerant rats. Additionally, hypothalamic H2S inhibition reversed endotoxin tolerance reestablishing fever, AVPO and plasma PGE2 levels without altering the absent plasma cytokines surges. Conclusion Endotoxin tolerance is not simply a reflection of peripheral reduced cytokines release but actually results from a complex set of mechanisms acting at multiple levels. Hypothalamic H2S production modulates most of these mechanisms. - 'Acta Physiologica, EarlyView. '