  The influence of ultra-high energy cosmic rays (UHECRs) and decaying dark
matter particles on the emission and absorption characteristics of neutral
hydrogen in 21 cm at redshifts $z = 10-50$ is considered. In presence of UHECRs
21 cm can be seen in absorption with the brightness temperature $T_b=-(5-10)$
mK in the range $z=10-30$. Decayng particles can stimulate a 21 cm signal in
emission with $T_b\sim 50-60$ mK at $z =50$, and $T_b \simeq 10$ mK at $z \sim
20$. Characteristics of the fluctuations of the brightness temperature, in
particular, its power spectrum are also calculated. The maps of the power
spectrum of the brightness temperature on the plane {\it wavenumber-redshift}
are shown to be sensitive to the parameters of UHECRs and decaying dark matter.
Observational possibilities to detect manifestations of UHECRs and/or decaying
particles in 21 cm with the future radio telescopes (LOFAR, 21CMA and SKA), and
to distinguish contributions from them are briefly discussed.
