PAY: $35-40 for a two-part, 3-4 hour study. ELIGIBILITY: 18-23 yrs old, NO smokers, NO birth control use (IUDs and Nuvaring OK), and NO prescription medicine use.MUST sign up for Part 1 and Part 2 four days later. TO SIGN UP: - Go to URL listed below under Additional Information - Make username and password (if you haven't before) - Look for Expt. 878