In fish, the endocrine system, especially corticosteroids pathway, strongly interacts with immune system. On the other hand, in vivo co-stimulation of both systems is not well documented. To better understand this interaction, we decided to evaluate the in vivo effects of both stimulation of the immune system and co-stimulation of both systems in Eurasian perch juveniles. Fish were injected either with 10mgkg(-1) LPS, or with a combination of LPS and 0.8mgkg(-1) cortisol or LPS and 0.08mgkg(-1) 11-deoxycorticosterone (DOC) and sampled 1, 3 or 7days after injection. LPS affected the immune system by increasing plasma lysozyme activity and blood neutrophils populations. During the same time-course, LPS decreased the proportion of a mixture of lymphocytes and thrombocytes in blood and TNF-? expression in spleen. Cortisol modulated the LPS-mediated response in TNF-? mRNA expression levels in spleen. Contrary to LPS alone, the association of LPS with DOC modulated the abundance of complement component 3 (C3) mRNA in spleen. On the other hand, LPS altered the corticotropic axis by decreasing mRNA expression levels of all corticosteroid receptors and of 11?-HSD-2 in spleen. Both corticosteroids injected were not able to balance these LPS-induced suppressive effects on corticosteroid receptors and 11?-HSD-2 expression levels in spleen. Contrary to LPS alone, the association of LPS with DOC modulated GR-1b expression in gills. These results indicated that LPS is a strong modulator of the corticosteroid receptors expression in spleen. Furthermore, we report for the first time a LPS-induced decrease of the mineralocorticoid receptor expression. Finally, corticosteroids were able to modulate the LPS-mediated response at the transcriptional level.