The effect of abscisic acid (ABA) on uptake of potassium ((86)Bb(+) or (42)K(+)) by Avena sativa L. coleoptile sections was investigated. ABA lowered the potassium uptake rate within 30 min after its application and inhibition reached a maximum (ca. 75%) after 2 h. The inhibition of K(+) uptake increased with ABA concentration over a range of 0.03 to 10 μg/ml ABA. At a higher K(+) concentration (20 mM) the percentage inhibition decreased. The percentage inhibition of K(+) uptake by ABA remained constant with external K(+) varied from 0.04 to 1.0 mM. After a loading period in 20 mM K(+) ((86)Rb(+)), apparent efflux of potassium was only slightly increased by ABA. Experiments in which growth was greatly reduced by mannitol or by omission of indole-3-acetic acid from the medium indicated there was no simple quantitative correspondence between ABA inhibition of coleoptile elongation and ABA inhibition of K(+) uptake. Chloride uptake was also inhibited by ABA but to a smaller degree than was K(+) uptake. No specificity for counterions was observed for K(+) uptake. Uptake of 3,0-methylglucose and proline were inhibited by ABA to a much smaller extent (14 and 11%) than that of K(+), a result which suggests that ABA acts on specific ion uptake mechanisms.