 The study evaluated the performance of peat and zeolite mixtures as substrates for cucumber seedling growth and crop yield in a greenhouse covered with double polymeric film at the Institute of Horticulture, Lithuanian Research Center for Agriculture and Forestry. Cucumber seedlings grown in peat zeolite substrates were shorter, had smaller leaf areas, and higher root masses than those grown in peat alone, but no significant difference was found in their dry mass. The addition of zeolite to peat substrate did not affect the photosynthesis pigment content in cucumber seedling leaves, but it increased the net assimilation rate. Physiological growth indices were lower in cucumber seedlings grown in peat zeolite substrates compared to those grown in peat alone, but higher yields were demonstrated by cucumbers grown in peat zeolite substrates. This article was authored by Jules Jankowski, Oscar Breseitite, Victorija Vasticate Kareen, and others.