3-Aminobenzamide (3-ABAM), a specific inhibitor of the nuclear enzyme ADP-ribosyl transferase (ADP-RT) has been previously shown to inhibit the differentiation of tracheary elements without affecting cell division (Hawkins and Phillips, 1983). This effect is confirmed here in cultured explants of pea as well as Jerusalem artichoke, and total inhibition of direct differentiation in cultures of gamma-irradiated immature artichoke tubers demonstrated. In studies on the timing of action of 3-ABAM, cells were found to be sensitive only during the 6 h period prior to the onset of visible differentiation, i.e., after the final mitosis when cells were known to be in the G1/G0 phase of the cell cycle. The duration of effectiveness of 3-ABAM was limited to a maximum of 3–4 d, apparently due to metabolic degradation of the inhibitor. Other reported ADP-RT inhibitors and their analogues were found to be toxic in this system. The blocking of differentiation by 3-ABAM was freely and rapidly reversed on removal, and by this means the maturation time for differentiating tracheary elements was estimated at 6 h. The potential of studies on ADP-RT activity, and its possible role in regulating DNA strand breakage and repair for advancing our understanding of the molecular mechanisms involved in xylem differentiation are discussed.