I've got bunch of web pages under one domain and several subdirectories; eg. domain1.com/site1, domain1.com/site2 and so on. There's tons of website, directory and forum links to those pages.

I want to change some sites to work under other domains I own, eg. domain1.com/site1 (and all files under it) to redirect (301) to domain2.com/site1, domain1.com/site2 to domain2.com/site2, and domain1.com root directory to redirect to domain3.com.

How can I do this with .htaccess?

*picks bellybutton lint, while drooling on keyboard*

I'm not sure how to do this with .htaccess...

Why would you want to do this at all? Is this a temporary redirect?

Anyway, here's what a quick search came up with... I didn't read the whole thing but it looks like what you're trying to do:

No it's a permanent change. My pages grew out of their original domain, some warranting their own domain, some better fitted under a new domain.

I set up now a simple .htaccess redirect, it works for most of the pages... Most places say it'd be better to use RewriteEngine but I'm not sure why.

Are the current sites indexed so prevalently in search engines that people are going to be finding the old domains for a long time?

Search engines won't be much of a problem, it's links from personal sites, forums and rarely updated directories that amount to lots of traffic. Only some 20% of my traffic is from search engines.

Have you tried messaging pocksucket? He's a really big nerd and may be able to help... I kind of think this is one of his areas of expertise... Kind of. But then I don't know d**k about what you're saying, so who knows. I'll recommend pocksucket, though.