Energy regulator Ofgem has launched an investigation into Scottish Power and Scottish and Southern Energy (SSE).
Ofgem said it has had a "formal complaint alleging abuse of a dominant position in the electricity generation sector arising from constrained capacity on the transmission network, as well as informal enquiries".
The probe is separate to the more wide-ranging investigation into the energy supply markets, which Ofgem launched on February 21st 2008.
SSE said: "No specific allegations have yet been put to SSE, but we are confident our actions in the electricity generation market have always been justifiable in economic terms and consistent with the operation of a competitive market.
"With any Competition Act investigation there is a process to be followed and we will cooperate fully with Ofgem throughout this process in order to bring it to a speedy conclusion."
The two own the Scottish transmission system between them and control most of the power stations in the country.
National Grid, which owns the transmission system in England and Wales, has to pay for use of the system in Scotland.