The inputs and outputs of potassium (K) and sodium (Na) were measured in six lambs prior to and during a 4-day period of acute dietary K restriction. During the predepletion period, urinary K accounted for 60.4% of the total K excreted. Renal K and Na excretion decreased during the initial stage of K restriction. Renal K excretion then continued to decrease and reached a minimum level after 40 h of K restriction, whereas fecal K excretion decreased only moderately from predepletion levels. A transient natriuresis, which occurred concurrently with the minimum level of urinary K, supported the hypothesis of the existence of a renal peritubular Na–K exchange mechanism. The delay in renal K conservation mechanisms, together with an obligatory fecal K excretion, caused an acute K deficit, estimated at 6.7–11.4%, in lambs fed a low K diet (0.032% K) for 4 days.