Changes in blood, urine and physical condition indices in 23 adult male black-tailed jackrabbits (Lepus californicus) with ad libitum feeding and 25% feed restriction were measured over a 2 wk period from 30 May to 12 June 1988. Feed restricted jackrabbits had (1) lower posttrial body weights and kidney fat indices, (2) higher femur marrow fat, serum bilirubin and cortisol concentrations, and adrenal cortex width, and (3) depressed immune function. No single index alone could best measure the nutritional status of these jackrabbits.