We report the detection of rapid variability in the hydrogen Balmer lines, Ca II H, K, b Mg I triplet at the chromospherically active stars 61 Cyg A, B in the subsecond range. Spectral observations of 61 Cyg A, B were performed with the MMCS spectrograph at the Cassegrain focus of the 2-m telescope at peak Terskol (R = 13000), with a grating spectrograph mounted on the 60 cm telescope at peak Terskol (R ≈ 100), and the 60 cm telescope of the Andrushivka Observatory (R ≈ 200) in the framework of the international campaign of observations of variable stars with the Synchronous Network of Telescopes. High resolution spectra were obtained at intervals of about 2 minutes. Low resolution spectra were obtained at intervals of two to three seconds. Spectral monitoring lasted from half an hour to two hours. We found that the chromospherically active dwarf stars 61 Cyg A, B, having a powerful spots on their surfaces exhibit rapid variations in the hydrogen lines, ionized calcium, Ca II H, K, chromospheric triplet b Mg I on a time scale of about 2 seconds to about 2 minutes. Variations in intensity of the lines range from 1.5 % in the Balmer lines to about 0.5 % in the lines of the triplet b Mg I. These variations are well described by the model of the shot noise in the form of random elementary pulses with the frequency of occurrence of a few fractions of events per second to a few events per second.