[Kirk]

I forgot to aerate prior to pitching.  When is it too late to aerate? 

[mabrungard]

It depends on how active your fermentation is.  If its still pretty lackluster, you can hit it with air or O2.  A secondary aeration is sometimes used in big beers to help boost the yeast count.  That is typically done between a half day and a day after pitching.

If you pitched a good starter and that yeast count was already high, then its possible you don't really have to aerate upon pitching.  If its a big beer, you'll want to do it.

[Hokerer]

I forgot to aerate prior to pitching.  When is it too late to aerate? 

You're probably safe up to 24 hours or so after pitch

[majorvices]

Aeration strengthens the cell walls so the cells can divide properly. As was mentioned, if you pitched a healthy starter it may very well not be necessary to aerate especially if you aerated the starter during propagation. Also, if you pitched dry yeast you probably don't need to aerate because the dry yeast cells are supposed to have their glycogen stores already built up. If, however, you only pitched a vial or smack pack you will definitely need to aerate or you will most likely have fermentation troubles because not only are you most likely underpitching but if your yeast is not extremely fresh then it has already started using those reserves and will not be in very good shape for fermentation.

[Kirk]

Thanks guys.  Since I had the choice, I thought it wiser to oxygenate rather than aerate, so instead of of ripping it with the drill and stirring rod, I used my diffusion stone and half empty bottle of oxygen. Makes for a good reason to always have a bottle of oxygen on hand.  At any rate, I feel better now.

Amazing, at 7 A.M. on a Sunday morning I post a question, and within 3 hours I get input from some of the top brewers in the country.  Good reason to be a member.