Abstract: The relationship between germination and melatonin applied during osmo- and hydropriming was studied in cucumber seeds. The proportion of nuclei with different DNA contents, the mean ploidy and the (2C + 4C = 8C)/2C ratio in unprimed and primed, dry and imbibed at 10°C seeds were established by flow cytometry. Thiobarbituric acid reactive substances and protein oxidation were also estimated. Melatonin and indole-3-acetic acid (IAA) concentrations in the seeds were determined using high-performance liquid chromatography with electrochemical detection. Being sensitive to chilling stress, seeds that germinated well (99%) at 25°C showed only 30% germination at 15°C, and almost no germination (4%) at 10°C. Hydropriming in water improved seed germination to 50–60% at 15°C and the addition of melatonin (25–100 m) also increased the rate of germination. Osmopriming in polyethylene glycol increased germination at 15°C to 78%, and 98% when combined with 50 m melatonin. Osmoprimed seeds germinated even at 10°C and reached 43%, and 83% when 50 m melatonin was applied. None of the treatments induced DNA synthesis, although during the first 24 hr of imbibition at 10°C the mean ploidy and the (2C + 4C = 8C)/2C ratio increased, which is indicative of the advanced Phase II of germination. Hydro- and osmopriming slightly decreased IAA content in the seeds in most of the cases; only hydropriming with 100 and 500 m melatonin increased it. Melatonin protected membrane structure against peroxidation during chilling, but excessive melatonin levels in cucumber seeds (∼4 μg/g fresh weight) provoked oxidative changes in proteins. There is still lack of information explained clearly the role of melatonin in plant physiology. This molecule acts multidirectionally and usually is alliged to other compounds.