The talks are at an exploratory stage and could yet break down because of regulatory hurdles. But sources close to the two companies said that PacifiCorp was ScottishPower's preferred US partner.
ScottishPower, which is valued at pounds 7.6bn, has never made any secret of its US ambitions. In the past it has contemplated a takeover of both Florida Progress and the Ohio-based utility Cinergy, which owns a half stake in Midlands Electricity.
PacifiCorp is seen as vulnerable to a bid, having parted company earlier this year with its chief executive, Fred Buckman, following the failure of the group's bid for Energy Group.
However, any bid would be fraught with complications. PacifiCorp, based in Portland, Oregon, operates in five different states and any takeover would have to be approved in each one.
A spokesman for ScottishPower refused to confirm that it was in talks with PacifiCorp. He added that whether ScottishPower took over or merged with an American utility, it was important that it had management control.
PowerGen is also still looking for a US acquisition, having abandoned plans to forge a "merger of equals" with the Texas-based utility Houston Industries. The merger fell apart because the two sides could not agree on valuation and who would run the combined group.
Ian Robinson, chief executive of ScottishPower, would run the combined group, although there would also be a role for Keith McKennon, PacifiCorp's chairman, who took over temporarily following Mr Buckman's departure in September.
He left after PacifiCorp was beaten in the long-running bid battle for Energy Group, owner of Eastern Electricity, by Texas Utilities.