Slices of submandibular glands of newborn and early post-natal rats took up K+ when incubated in vitro in an enriched, oxygenated Krebs-Ringer bicarbonate solution. This uptake of K+ was observed when the slices were transferred to K+-containing medium after pre-incubation in K+-free medium, when the previously stimulated cholinergic or alpha-adrenergic receptors were blocked with appropriate antagonists or when the Ca2+ in the medium was chelated with EGTA 2 min after stimulation with carbamylcholine. The extent of K+ uptake was larger in the youngest (1-day-old) animals in all these experiments. Uptake of K+ was inhibited by 1 mM ouabain. This inhibition was not immediate and varying rates of K+ uptake were observed for 2-5 min after addition of the glycoside. It is concluded that: (1) K+ uptake can be elicited in the developing submandibular glands of newborn and early post-natal rats; (2) this response is sensitive to ouabain and is likely to depend, therefore, on the presence of a Na+, K+ ATPase in the immature gland; (3) changes in ouabain-sensitive K+ uptake with age suggests that the activity of the Na, K ATPase varies as the gland matures in the immediate post-natal period. As the Na+, K+ ATPase is thought to provide a driving force for the transport of ions underlying salivary secretion, the findings suggest that this mechanism is present in the early stages of post-natal development of the rat submandibular gland.