Fannie Mae said Friday morning that its mortgage investment portfolio grew at a 17.3 percent annual rate in October, largely the result of a $7.1 billion purchase of a Lehman Brothers credit facility. The investment portfolio reached $732.3 billion, the GSE reported, which was within the portfolio cap mandated by the Office of Federal Housing Enterprise overshight. Click here to see the monthly summary. Purchase commitments dropped precipitously in October, however, falling to $26 billion from $40.2 billion in September — a sign of slowing origination activity and a harbinger of potentially significant drop in future securities issuance volume. Fannie also reported that total residential delinquences continued their upward trend, reaching their highest levels since at least November 1997. Serious delinquency rates among conventional single-family mortgages, defined as mortgages 60 or more days past due, increased 7 basis points reached a total 0.78 percent of loans. Delinquencies among Fannie credit-enhanced mortgage holdings reached 2.18 percent in October, the highest level in at least five years. For more information, visit http://www.fanniemae.com.