Following a previous observation that moderate physical training (running) of rats did not impair T-cells, in this study moderately trained Wistar rats were run to exhaustion on 2 consecutive days: in one case (T-dex) this was preceded by an intraperitoneal injection of 0.5 mg.kg-1 of dexamethasone (dex) and in the other case there was no prior injection (T). Similarly one group of sedentary control rats, was injected with dex (C-dex) and the other group was not (C). Rats were killed 24 h after the last treatment (dex, exercise). Compared with the C rats, the T rats exhibited a decreased number of thymocytes (75%), in particular CD4+CD8+ thymocytes and splenocytes (55%), notably CD4+CD8- splenocytes (P < 0.01). Also noted in the T rats was a lower (45%) in vitro (+mitogen) percentage of IL2r+CD4- splenocytes (expressing the IL2 receptor), and reduced (40%, P < 0.01) or unchanged in vitro production of T-cell growth factor (TCGF) by splenocytes or blood mononucleated cells (BMC), respectively. The dex decreased the number of thymocytes and splenocytes in the same way in T-dex rats (compared to T rats) and in C-dex rats (compared to C rats, P < 0.01). In T-dex rats compared with C-dex rats, on the other hand, dex had little effect on in vitro TCGF production by BMC, and no effect on other in vitro parameters. These results would indicate that physical exhaustion was responsible for an alteration in T-cells in the moderately trained rat. This alteration was in part enhanced by dex.