The early time course of adaptation to large step increases in K intake was examined in sheep and rats. Fifteen 3-day experiments were performed on four mature ewes. They received on each day a single meal (730-930 meq K/day) and on days 2 and 3 a rumen KCl supplement (600 mM/day). Adaptation to the changed intake occurred within 47 h and was defined by the ratio of urinary K/K intake approximating normal preloading ratios. K excretion did not correlate significantly with plasma K or with Na excretion. Three groups of four rats, body wt 210 g, were studied over 19 days. Four rats fed a basal diet excreted 1.96 +/- 0.04 (n = 19) meq/day K. For four rats, the basal diet was supplemented with KCl on days 5-15, during which time K excretion was 9.34 +/- 0.36 (n = 11) meq/day; four rats with a higher KCl supplement on days 5-15 excreted 15.37 +/- 0.69 (n = 11) meq/day K. For rats, adaptation to increased and decreased intake was rapid, occurring on the first day of changed intake when urinary K excretion approximated intake. The rapid K adaptation was contrary to the generally accepted, but experimentally unverified, view that adaptation is a chronic process requiring 1 or more weeks to develop.