Both adult and suckling Richardson's ground squirrels (Spermophilus richardsonii) were susceptible to subcutaneous exposure with western equine encephalomyelitis (WEE) virus, but the virus was more virulent for sucklings than adults. In sucklings, the incubation period was from 4 to 5 days, followed by apparent signs of central nervous system (CNS) involvement. Death occurred 10 to 13 days postexposure. In adults, infections were inapparent or acute with typical signs of CNS involvement similar to those observed in sucklings. In both age groups, brain and lymph nodes were the most frequently involved tissues. The highest titres of virus were recovered from brain, sections of which also showed the most marked histological changes. Lesions in the brain included multifocal vasculitis, perivascular edema, perivascular cuffing, focal or diffuse gliosis, parenchymal hemorrhage, meningitis with infiltration of mononuclear cells, neuronal degeneration, and occasional demyelination. For both age groups, viremias were detected for 3 to 5 days with a maximum virus titre of 4 to 6 logs, a sufficient time and magnitude to infect numerous mosquitoes, further supporting the hypothesis that S. richardsonii may serve as an amplifying host of WEE virus in the prairie provinces.