May a person perform or otherwise participate in experiments on live animals which will cause the animals pain? I would assume this is forbidden under tzaar baalei chayim. But perhaps there is a heter for scientific purposes?
Rabbi Yissochar Frand has a tape on inflicting pain to animals.
Generally the prohibition is on inflicting pain for no good reason, and legitimate medical research is a good reason. He quotes a responsum -- I believe it's the Shvut Yaakov -- about a doctor testing a medicine on a dog or cat first, who allowed it, but said it's ideal not to do the procedure yourself, as you'd be training yourself to be insensitive to suffering. (I corresponded with Rabbi Yehuda Herzl Henkin shlit'a about this, and he didn't see this latter point as an issue.)
Rabbi Frand concludes he's not sure whether cosmetics are considered enough of a cause; Rabbi Yitzchok Breitowitz gave a public lecture a few years ago in which he felt they weren't.
(Mind you, some neighborhoods/cultures have much stronger expectations about women's cosmetics. Rabbi Dovid Miller of Yeshiva University's Gruss Kollel in Jerusalem has an mp3 in which he discusses a mikva-related heter involving makeup, with a comment to the effect of "I don't know, in my world if you don't wear makeup whatever, but in the Charedi yeshivish world, if she's not totally painted, something's wrong!")