The ten metopes on the east side depict the Labours of Heracles. The four easternmost metopes on the north and south sides depict the Labours of Theseus.
After Pericles came to power in Athens, he began a campaign to transform the city into the centre of Greek power and culture. Construction on the temple of Hephaestus (Theseum) started in 449 BC, and may not to have been completed for some three decades afterwards, funds and workers having been redirected towards the Parthenon. The western frieze was completed between 445–440 BC, while the eastern frieze dates to 435–430 BC, the dates being determined largely on stylistic grounds. It was only during the Peace of Nicias (421–415 BC) that the roof was completed and the cult images were installed. The temple was officially inaugurated in 416–415 BC.
Olsen, E. (1938). "An Interpretation of the Hephaisteion Reliefs". AJA 42 (2): 276–87.