The aim of the study was to analyse the quality of extruded cereals enriched with plant by-products and to obtain fermented drinks from production rejects. Extrusion was performed with co-rotating twin-screw extruder (compression ratio 8:1) at MILZU Ltd. from rye and oat flour (80:20, control samples) with addition of apple (ABF), carrot (CBF) and pumpkin (PBF) by-product flour in various amounts (10%, 15% and 20%). Naturally fermented kvass production process was used for non-alcoholic fermented beverage production. Total dietary fibre (TDF), textural properties and sensory features of extruded products after addition of by-products (BP) was determined. Dry matter, active acidity and sensory properties were analysed in fermented beverages. The obtained results showed a 12-55% increase in TDF of extruded cereals (11.8 g 100 g-1) after addition of plant by-products. All extruded samples with BP showed lower hardness levels than control (35.55 ± 2.95 N); samples with PBF were the least hard (P < 0.05). Highest taste and aftertaste scores using 5-point hedonic scale were given to samples with addition of 15% and 20% ABF, which also showed high consumer acceptance. With regards to fermented drinks, the highest dry matter content was found in PBF and ABF drink, 8.1 ± 0.1 and 7.0 ± 0.1, respectively. Sensory evaluation of fermented beverages showed that the intensity of flavour, acidity and aroma was most pronounced in sample with ABF, whereas colour was most pronounced in sample with PBF. In order to reduce production costs, it is possible to use production rejects of extruded cereals enriched with plant by-products to obtain new products.