From Project Athinai
During the 6th century BC, the Athenians created three gymnasia outside the city walls. These gymnasia were intended for the military training of the ephebes. The Kynosarges was one of these three famous gymnasia. In 421 BC, it was still used as a gymnasium. It was located to the southeast of Athens along the southern bank of the Ilissos river. A road connecting the gymnasium to the Diomeian Gate, crossed the Ilissos immediately upstream of the Kallirrhoe cisterns.
The gymnasium probably consisted, at least, of a precinct dedicated to Herakles, a dromos (racetrack) and a palestra.
Shortly after our period of reference (421 BC), Antisthenes, the founder of the Cynic school of philosophy, used to teach his followers at the Kynosarges.
Flacelière, Robert, La vie quotidienne en Grèce au siècle de Périclès, Hachette, 1959
Travlos, John, Pictorial dictionary of Ancient Athens, Books that matter, New York, 1971, p. 340, 345