Rats were exposed for 1,3,13,52 or 78 wk to air or a simulated urban profile of O3 designed to mimic diurnal exposure patterns frequently seen in worst case summer environments. Daily exposures consisted of a background level of 0.06 ppm for a period of 13 h, a broad exposure spike rising from 0.06 ppm to 0.25 ppm and returning to 0.06 ppm over 9 h and a 2 h downtime. Integration of the spike portion of the exposure pattern was equivalent to a 9 h square wave of 0.19 ppm. Rats were exposed to the profile 5 days/wk; weekend exposures were to background levels only. Spleens were removed and blood was drawn at the end of the exposure periods. O3 exposure had no effect on NKC activity, nor were any O3-related changes in mitogen responses or histopathology noted. Spleen cell mitogen responses, but not NKC activity, were significantly depressed, presumeably as a result of age, following the 52 and 78 wk exposures. Effects of age were apparent in the PBL responses to mitogens following the 13 wk as well as the 52 and 78 wk exposures. For comparative purposes, effects of a single, 3 h, exposure to 1 ppm O3 on spleen cell responses to the same mitogens were also determined 24, 48, and 72 h after exposure; there were also no effects due to the acute exposure.