Cytokinin free bases, ribosides and 9-glucosides were measured in stem node cultures of potato (Solanum tuberosum L. cv. Ulster Sceptre) in the presence or absence of 1 μM jasmonic acid (JA) to examine whether or not their changed levels were part of the JA-induced growth response. The enhanced growth response in JA-treated plantlets included: expanded root systems, extended leaf areas, increased number of nodes, and enlarged stem diameters. The protein analysis revealed a substantial decrease in a 62-kDa polypeptide. On a dry weight basis, the levels of ribulose-1,5-biphosphate carboxylase/oxygenase (RuBP carboxylase, EC 18.104.22.168) and chlorophylls a and b were constant. The total concentration of endogenous cytokinins remained virtually the same in control and treated plantlets; but in JA-treated plantlets the amount of cytokinin free bases and cytokinin 9-glucosides decreased. In addition, the level of cytokinin ribosides was elevated. The ratio between active and inactive cytokinins increased from 1.2 to 2.1, which correlates with the enhanced growth of potato plantlets grown on 1 μM JA. Thus the observed growth and developmental changes may be a consequence of the measured altered cytokinin level. However, significant morphological alterations of the potato plantlets treated with JA may also be a result of the changed critical cytokinin concentration or critical ratios of cytokinins to auxins and JA, rather than their absolute concentrations.