GM saw US sales in May fall by 5.5% on the same month a year earlier, while Ford's dropped by 3%.
May's fall in sales was the 12th straight decline for Ford and the fourth in a row for GM. GM made a loss of $1.1bn (£605m) in the first quarter of this year, while Ford saw its profits fall by 40% to $1.2bn (£660m) during the same period.
Each is also to introduce a new range of price cuts for this month, with GM promising to offer buyers the same discounts it offers employees, in addition to other incentives such as cash-backs.
Motor analysts said the price reductions would inevitably give sales a short term boost.
Overall annual US car sales totalled 16.7m in May, down from 17.7m in May 2004.