Tax-exempt money market funds gave up two-thirds of the impressive gains they generated last week, reporting $4.21 billion in net outflows as total net assets settled at $288.55 billion in the week ending Jan. 14, according to the Money Fund, a service of iMoneyNet.com.

The outflows contrasted with on the $6.18 billion of inflows the funds netted the prior week, which almost doubled the $3.22 billion the funds accumulated in the week ended Dec. 31 and was higher than inflows for any week in 2012.

The average, seven-day simple yield for the 1,040 reporting tax-exempt money funds was unchanged at 0.01% from the prior week, while the average maturity remained at 32 days.

Outflows also derailed the hearty gains that taxable money funds earned last week. Total net assets for the 1,040 taxable reporting money funds fell by $11.95 billion in the week ended Jan. 15 to $2.397 trillion — erasing more than half of the $21.24 billion of inflows amassed last week.

The average, seven-day simple yield for the taxable funds was unchanged at 0.02% from the previous week, while the average maturity increased by one day to 47 days.

Overall, the combined total net assets of the 1,471 reporting money funds decreased by $16.16 billion to $2.686 trillion in the week ended Jan. 15. The outflows come on the heels of $27.43 billion of inflows in the prior week when total net assets closed at $2.702 trillion.