In etiolated seedlings of Pisum sativum and leaves of Arabidopsis thaliana, in vivo ethylene treatment resulted in an increase in in vitro phosphorylation of 17 kD (P. sativum) or 16 and 17 kD (A. thaliana) polypeptides. These polypeptides were identified as nucleoside diphosphate kinase (NDPK) based on both biochemical properties and interaction with antibodies against NDPK from P. sativum. Using the receptor-directed antagonist of ethylene action 2,5-norbornadiene and the ethylene-insensitive mutants of A. thaliana etr1-1 and eti5, ethylene specificity and receptor dependence of NDPK phosphorylation have been demonstrated. In pea epicotyls, ethylene treatment also led to increase in nucleoside transferase activity unlike in A. thaliana leaves. The increases in nucleoside transferase activity and NDPK phosphorylation were very rapid and transient. The results suggest a role for NDPK as a possible component of the ethylene signal transduction chain.