The effects of postharvest storage duration and temperature on endogenous cis-zeatin (cis-Z) and cis-zeatin riboside (cis-ZR) levels in potato (Solanum tuberosum L.) tubers were determined in relation to tuber bud dormancy. The tubers used in these studies were completely dormant for at least 81 days of storage. Thereafter, tuber bud dormancy diminished gradually and after 165 days of postharvest storage, the tubers were completely non-dormant. Immediately after harvest, endogenous levels of cis-Z and cis-ZR were approximately 25 pmol (g fresh weight)−1 and 8 pmol (g fresh weight)−1, respectively. In tubers exiting dormancy but stored at a growth-inhibiting temperature (3°C), endogenous levels of cis-Z rose over threefold after 25 days of storage and remained elevated for the duration of the study. Levels of cis-ZR remained essentially constant during this same period. In tubers transferred to a growth permissive temperature (20°C) prior to use, the rise in endogenous cis-Z was less dramatic and more protracted; increasing twofold after 53 days of storage. No change in cis-Z riboside content was observed in these tubers during this period. Dose-response studies using either cis-Z or trans-Z demonstrated a time-dependent increase in cytokinin sensitivity during postharvest storage. Immediately after harvest, dormant tubers were insensitive to both zeatin isomers. Thereafter, tubers exhibited a dose-dependent increase in premature sprouting following injection with either cytokinin isomer. After injection into dormant tubers, cis-[8-14C]-zeatin was metabolized primarily to adenine/adenosine and cis-Z riboside. Seven days after injection, less than 10% of the recovered radioactivity was associated with trans-ZR. These results are consistent with a role for endogenous cis-Z (and its derivatives) in the regulation of potato tuber dormancy.