In excised Avena leaves, depending on the duration of treatment, abscisic acid (10−5 M) had two distinctly different effects on the level of individual nucleases. In short-term experiments (3-h treatment, abscisic acid increased the level of a relatively purine (guanine)-specific ribonuclease, in comparison with the water control. Accumulation of the abscisie acid-induced ribonuclease, however, levelled off rapidly during incubation and the amount of the enzyme approached a plateau in about 6 h. As the accumulation of this ribonuclease became retarded, abscisic acid induced a striking increase in the level of another nuclease, an enzyme non-specific in relation to the sugar moiety but exhibiting a relative adenine specificity. This latter nuclease also was shown to accumulate slowly in intact Avena leaves during ‘natural’ senescence. The Avena leaves contain, in small concentrations, a chromatographic variant of the sugar non-specific nuclease. This minor variant, despite its identical enzymological properties, was found to be physiologically different from the main component in that its concentration did not depend on the age of the tissues and was not affected by abscisic acid.