The incident took place near the village of Cerveny Kamen at about 0830GMT, Zuzana Farkasova, a spokeswoman for Slovak firefighters told AP.
An ambulance and at least three helicopters were deployed to the crash site, Andrej Hirjak, a spokesman for the rescuers said.
The seven victims were confirmed by the Slovak Interior Ministry.
Both aircraft reportedly had up to 40 people on the manifest list before takeoff, according to local news outlet TA3. The passengers are said to be paratroopers, most of whom reportedly bailed out before the collision, according to Denník newspaper.
The two sport aircraft, an AN-2, a Soviet-made single-engine biplane, and an L-410, a Czech twin-engine aircraft, were practicing for a parachute show to be held this Sunday in the village of Slavnica.
The paratroopers were members of Slovak National Aero Club Jan Mikus, according to TA3.
Witnesses estimated that the planes were flying at an altitude of about 1km at the time of the crash.
“I heard the crash and ran [to the site of the accident]. I saw one of the aircraft falling to the ground and people jumping from it,” one witness told Dennik newspaper.