The invention relates to a compound which is, 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]adenosine, which is a selective A2A adenosine receptor agonist in myocardial imaging. The new polymorph of 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]adenosine (designated as polymorph E) is characterized by an X-ray diffraction pattern of X-RPD showing the following reflections at 2 Theta=5.8°, 12.3°, 15.9°, 17.3°, 20.5°, 22.6°, 23.6°, 27.7°, and 29.2°; and further characterized by a DSC scan showing marked endotherm in the range of 258 to 264° C.; and further characterized by a specific IR spectra. The invention further relates to a method of preparing the polymorph by recrystallization from other polymorphic forms of 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]adenosine by a procedure comprising the following operations: mixing of 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]adenosine with a polar aprotic solvent, preferably with dimethylsulfoxide, and heating to form a saturated solution; cooling of the saturated solution with formation of a turbid solution of 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]adenosine; addition of the turbid solution of 2-[4-[(methylamino)carbonyl]-1H-pyrazol-1-yl]adenosine to a protic solvent, preferably methanol, with separation of a gel-like precipitate; heating of the separated gel-like precipitate in the protic solvent to a boil with formation of a suspension of polymorph E; and cooling of the suspension, isolation and drying of polymorph E.