Six potato varieties belonging to four different sub-species were submitted to drought stress during tuberization under controlled field conditions, resulting in contrasting responses of the genotypes to water stress with yield losses varying between 27 and 75%. In all clones free proline accumulated under drought, however, proline levels increased earlier in drought-susceptible varieties than in more tolerant ones. The expression of two key genes in proline metabolism, Δ1-pyrroline-5-carboxylate synthase and proline dehydrogenase, was monitored in the leaves of the experimental plants by real time PCR 23 and 42 days after drought onset. Expression of both enzymes did not correlate with the proline levels found in leaf tissue indicating that mechanisms other than transcription participate in the regulation of proline accumulation in potato leaves. © 2006 Polish Academy of Sciences.