Objectives: Cohabitation with Ehrlich tumor-bearing (ETB) mice induced behavioral, neurochemical, hormonal, and immune effects in the conspecifics as a consequence of stressinduced activation of the sympathetic nervous system (SNS) with catecholamine release. In the current study, the nonspecific ??-AR blocker d,l -propranolol and the specific ?? 2 -AR blocker ICI-118.551 were employed as pharmacological tools to assess the extent to which catecholamines participated in the effects induced by cohabitation with ETB mice. Methods: Two experiments were performed, 1 with d,l -propranolol treatment and the other with ICI-118.551. One mouse in the experimental group was called the ???companion of the sick partner??? (CSP) since it was forced to live in the same cage with 2 (experiment 1) or 1 (experiment 2) cage mate that had been i.p. injected with 5 ?? 10 6 Ehrlich tumor cells. Results: The d,l -propranolol treatment, but not the ICI- 118.551 treatment, attenuated the effects of cohabitation with 2 ETB mice on both open-field behavior and the hypothalamic levels and turnover rate of norepinephrine. The 2 ??-AR blockers were unable to change the serum corticosterone levels and adrenal weights of the CSP mice; however, these drugs abrogated the effects of cohabitation on neutrophil oxidative burst and phagocytosis. Finally, an increase in the 5-HT turnover rate was observed in the olfactory bulb of CSP mice compared to their respective controls, an effect that was not modified by ??-AR blockade. Conclusion: These results confirm and strengthen our hypothesis that the SNS is involved in the effects induced by cohabitation with ETB mice and point towards ?? 2 -AR participation in the immune effects analyzed.Funda????o de Amparo ?? Pesquisa do Estado de S??o Paulo (FAPESP)Conselho Nacional de Desenvolvimento Cient??fico e Tecnol??gico (CNPq)FAPESP: 01/50481-0; 09/51886-3CNPq: 300764/2010-3