Stress has been associated with reports of both greater severity and prolongation of diseases in patients with the infectious origin as well as other immune-mediated diseases. Withania somnifera, an Indian medicinal plant used widely in the treatment of many clinical conditions in India, was investigated for its anti-stress properties using BALB/c mice subjected to chronic stress. The study aimed to investigate chronic stress-induced alterations on Th1 lymphocyte subset distribution and corresponding cytokine secretion patterns. Oral administration of chemically standardized and identified aqueous fraction of W. somnifera root (WS) at the graded doses of 25, 50, 100 and 200 mg/kg p.o. caused significant increase in the stress-induced depleted T-cell population and increased the expression of Th1 cytokines in chronically stressed mice. © 2006 Elsevier B.V. All rights reserved.