We could log quite a few overtime hours just talking about the ins and outs of the complex overtime regulations. But the short answers to your two questions are "yes" and "no," respectively -- at least under federal law.

"Yes," your employer can require you to work overtime and can fire you if you refuse, according to the Fair Labor Standards Act or FLSA (29 U.S.C. § 201 and following), the federal overtime law. The FLSA sets no limits on how many hours a day or week your employer can require you to work. It requires only that employers pay employees overtime (time and a half the worker's regular rate of pay) for any hours over 40 that the employee works in a week. (Some states' laws, however, give employees more rights than the FLSA does, so check with your state department of employment or labor to be sure.)

And "no," your employer doesn't have to pay you overtime if you work more than eight hours in a day. The federal law is interested only in weeks, not days. As long as you work fewer than 40 hours in a week, you aren't entitled to overtime. (But again a few states, such as Alaska and California, require employers to pay workers overtime if they work more than eight hours a day.)

To learn more about overtime, including the rules in your state, check out Your Rights in the Workplace, by Barbara Kate Repa (Nolo).