The effect of magnetic fields on biological systems has not been completely identified. In the reference literature, there is a description of the effect of magnetic field on seeds (a pre-sowing stimulation) and animal organisms. There are a few research studies referring to the effect of those fields on the plant vegetation including the development of fruits during their growth and ripening. Therefore, the objective of the present research was to determine the impact of constant and slowly changing magnetic fields on some selected characteristics of apples. Variable magnetic fields were applied; their magnetic induction was between 5 mT to 100 mT at a frequency of 50 Hz and between 50 μT and 150 μT at a frequency ranging from 10 Hz to 100 Hz. Also, constant magnetic fields were applied that ranged between 5 and 100 mT. The three year field experiments resulted in a 7 % increase in the content of total extract in fruits stimulated by a slowly changing magnetic field that showed an induction level of 100 μT. Owing to practical reasons, this magnetic field was found to be optimal. The most significant changes found after the stimulation performed referred to the levels of two basic monosaccharides: fructose and glucose.