Samsung, Hyundai and six other firms admitted giving millions of dollars to funds linked to President Park Geun-hye, but denied seeking favours.
Samsung admitted to giving the daughter of Ms Park`s friend an expensive horse.
Parliament is due to vote on Friday on Ms Park`s impeachment over her involvement in the scandal.
Massive protests have been held in recent weeks demanding her resignation.
The executives are being questioned by a cross-party committee of lawmakers. The panel has no power to punish but its chairman has said the hearing is a place for apology.
One of the corporate bosses acknowledged that it was difficult for firms to say no to government requests.
"It`s a South Korean reality that if there is a government request, it is difficult for companies to decline," said Huh Chang-soo, head of the GS Group and chairman of the Federation of Korean Industries lobby group.
The conglomerates all gave large donations to non-profit foundations operated by Choi Soon-sil, a close confidante of Ms Park.
Ms Choi has been charged with coercion and attempted fraud.
South Korean firms face rare corruption hearing
06 December 2016
Read: 1151
South Korean lawmakers are questioning leaders of the country`s biggest businesses in a rare televised hearing as part of a huge corruption inquiry.
_1513838881.jpg&h=190&w=280&zc=1&q=100)
