Intercosmos 25 was part of the APEX (Active Plasma Experiment) program to investigate the effects of streams of electrons and beams of plasma on the ionosphere and magnetosphere. The spacecraft was cylindrical, measuring 1.8 m long, 1.5 m in diameter. Eight solar panels extended in petal-like fashion from one end. It weighed about one metric ton at launch. A Czechoslovakian subsatellite, Magion 3, was deployed from Intercosmos 25 on December 28.

The Magion 3 spacecraft was manufactured as the subsatellite of the APEX (Intercosmos 25) spacecraft. The APEX project had the following primary objectives: Simulation and initiation of aurora and radio frequency radiation in the auroral area; study of the dinamics of modulated beams and plasmoids in the near - earth plasma; study of the nature of electrodinamic relationship of electromagnetic waves in magnetosphere and ionosphere; determination of radio emission characteristics of modulated beams of charged particles and plasmoids; search for non-linear wave structures of the electromagnetic soliton type in disturbed environment. The Magion 3 scientific payload was almost the same as for Magion 2.