We have previously shown that intermittent hypoxia (IHx, 10% O(2), 60 min/day) leads to an increase in the splenic alpha2-adrenoceptor response and results in a splenic contraction-induced reversible increase in hemoglobin concentration ([Hb]). In the present study, we determined whether IHx of shorter duration (15 min/day (15-min) and 30 min/day (30-min)), produced this phenomenon in rats. A significant increase in [Hb] during hypoxia was observed in both the groups, but its magnitude was larger in the 30-min IHx rats. Even when the cumulative exposure time (time/dayxdays) was shorter, the [Hb] increase was larger in the rats with longer daily hypoxic exposure. The alpha2-adrenoceptor antagonist yohimbine abolished the [Hb] increase of 15- and 30-min IHx. The increase in [Hb] following administration of the alpha2-adrenoceptor agonist oxymetazoline was also higher in 30-min IHx; indicating that the higher [Hb] produced by longer daily hypoxic exposure times is the result of increases in alpha2-adrenergic-receptor response of greater magnitude. In conclusion, IHx for periods as short as 15 and 30 min/day increases the splenic alpha2-adrenoceptor response and its magnitude reaches the maximum value depending on the daily hypoxic exposure time. A reversible increase in [Hb] constitutes a useful mechanism that protects organ oxygen supply during hypoxic episodes of variable duration and intensity.