(5) Retirement Plans

We sponsor programs that provide retirement benefits to most of our employees. These programs include defined benefit pension plans, defined contribution plans and postretirement healthcare plans. Key terms of our retirement plans are provided in our Annual Report. Our retirement plans costs for the three-month periods ended August 31 were as follows (in millions):

	2017		2016
Defined benefit pension plans	$	37	$	58
Defined contribution plans		127		119
Postretirement healthcare plans		19		19
	$	183	$	196

Net periodic benefit cost of the pension and postretirement healthcare plans for the three-month periods ended August 31 included the following components (in millions):

	U.S. Pension Plans						International Pension Plans						Postretirement Healthcare Plans
	2017			2016			2017			2016			2017		2016
Service cost	$	170		$	160		$	23		$	20		$	9	$	9
Interest cost		279			282			12			11			10		10
Expected return on plan assets		(406	)		(375	)		(11	)		(11	)		—		—
Amortization of prior service credit and other		(30	)		(30	)		—			1			—		—
	$	13		$	37		$	24		$	21		$	19	$	19

Contributions to our tax-qualified U.S. domestic pension plans (“U.S. Pension Plans”) for the three-month periods ended August 31 were as follows (in millions):

	2017		2016
Required	$	—	$	—
Voluntary		250		250
	$	250	$	250

In September 2017, we made $250 million in required contributions to our U.S. Pension Plans.