Three field experiments were established in 1996-2003 in order to determine the effects of term of harvest and stage of maturity on biological value of leek, zucchini and kohlrabi. The results of experiments showed that delay the harvest date associated with considerable increment of crop yield caused the enhancement of dry matter, total and reducing sugars in leek and kohlrabi. In zucchini the fruits of smaller size contained higher amounts of dry matter and similar sugars like more developed. Advanced term of harvest appeared to be favourable for vitamin C, phosphorus and potassium content in vegetables. Changes in magnesium and calcium concentration under influence of the stage of maturity were highly dependent on plant species. Plants of kohlrabi and leeks harvested at later stage of maturity contained lower level of nitrates, but in zucchini there was observed the increment of this compound in fruits of a bigger size at harvest.