Katie Couric's sudden rise to the top of the evening-news ratings heap is quickly running out of helium.

Between opening night - last Tuesday - and the end of the week - last Friday - the newscast has lost nearly half its audience.

When ratings were released early yesterday, it appeared at first that Couric had dropped to third place on Friday. But those early reports proved to be wrong.

While she remained No. 1 on Friday, Katie had settled down to a more sobering 7.4 million.

That's the closest Couric has come so far to CBS' typical ratings before she arrived.

No one at CBS is crying yet. Couric's was the top-rated newscast for the week, the network's first weekly win in five years.

CBS News insiders blamed Friday's dropoff on its lengthy coverage of the U.S. Open. As a lead-in for the "CBS Evening News," it delivered a smaller audience than local news.

As well, Friday traditionally draws fewer viewers to television.

As for Katie's sudden ratings bump - and fall to earth - CBS officials say long-term changes in the ratings race are usually "glacial."

For the week, the Katiecast averaged 10.2 million viewers.