With the widespread use of online questionnaires, motivating people to participate in web-based studies has become crucial to research success. Much of web-based research is conducted using online panels. This experiment examined the long-term effect of 6 rewards on response quantity in an online panel. All 3,200 newly recruited members of a noncommercial online panel were invited to 7 study waves. One half of the participants were offered a result summary in each wave; the other half was not. In addition, 1 of 6 rewards was promised for participation in each wave: no reward, money via bank transfer, money via PayPal, redeemable loyalty points, donation to a predetermined charity, or donation to a selectable charity. Measures of response quantity were attrition, response, and retention. Latent growth curve models revealed incentive effects on the intercept (i.e., baseline in Wave 1) and on the slope (i.e., change over the waves). Neither incentive influenced attrition and retention. PayPal and donations lowered baseline response. Result summary reduced the slope of response over the 7 waves. To maximize response quantity, researchers and practitioners should avoid using nonmonetary incentives such as donations and result summaries. When considering incentive costs, the findings call into question the usefulness of offering any incentive to boost response quantity, at least in noncommercial online panels. The present research has covered much ground in that it contrasted 6 rewards over time and crossed the rewards with the (non)offer of a summary. Explaining these differences remains to future experiments that trade breadth for depth.