1. The purpose of this study was to evaluate the roles of chronic hypokalaemia and of aldosterone in K+ transport in the medullary collecting duct. Renal clearance and duct transport measurements were made before and after KCl infusion in three groups of animals: normal rats on a regular K+ diet (group I), and sham-operated (II) or adrenalectomized rats (group III), both on a low K+ diet. 2. Only the sham-operated animals on the low K+ diet became hypokalaemic. They also had the lowest rate of K+ excretion at the time of study. Adrenalectomized rats were normokalaemic, and had an intermediate rate of K+ excretion. After the acute KCl infusion, kaliuresis remained significantly depressed in the rats which were previously hypokalaemic. In contrast, K+ excretion rates in response to K+ infusion were high and not significantly different in both previously normokalaemic groups, independent of the presence of the adrenal glands. 3. In the medullary collecting duct, before the KCl infusion there was no net K+ transport in either normokalaemic group (I and III). However, after the KCl infusion there was significant K+ secretion in both of these groups (32% and 22% of total urinary excretion, respectively). In contrast, the sham-operated hypokalaemic rats on the low K+ diet had a small absolute, but large fractional K+ reabsorption (64% of delivered load) in the medullary collecting duct. With KCl infusion in this group, K+ delivery to the medullary duct increased, but absolute reabsorption along the duct was maintained, resulting in a fractional reabsorption of 28% of delivery.(ABSTRACT TRUNCATED AT 250 WORDS)