Modeling a concentration curve with secondary peaks?

I need help for modeling a concentration curve with secondary peaks. What models could I use? I tried two compartment models, but it couldn't capture the secondary peaks. Suggestions or recommendations are highly appreciated. Thanks. Dr. Xiaoli Shirley Hou Senior Biometrician, Biometrics Research Merck Research Labs WP53-120 West Point PA 19486 Phone (215) 652-7498 (first week of each month) Phone (540) 381-9599 (rest of time) Fax (215) 993-1835 ------------------------------------------------------------------------------ Notice: This e-mail message, together with any attachments, contains information of Merck & Co., Inc. (One Merck Drive, Whitehouse Station, New Jersey, USA 08889), and/or its affiliates (which may be known outside the United States as Merck Frosst, Merck Sharp & Dohme or MSD and in Japan, as Banyu - direct contact information for affiliates is available at http://www.merck.com/contact/contacts.html) that may be confidential, proprietary copyrighted and/or legally privileged. It is intended solely for the use of the individual or entity named on this message. If you are not the intended recipient, and have received this message in error, please notify us immediately by reply e-mail and then delete it from your system.