The development and preliminary evaluation of the Computerized Adaptive Testing System (CATSYS), a new testing package for IBM-compatible microcomputers, are described. CATSYS can be used to administer and score operational adaptive tests or to conduct on-line computer simulation studies. The package incorporates several innovative features, including skip and review options, content balancing, item eligibility flagging, mastery testing, self-adapted testing, and listening items. Two hundred and five examinees took tests produced by CATSYS (items were from the Iowa Tests of General Educational Development) and completed questionnaires about the test. Results indicate that the examinees: (1) preferred CATSYS-produced tests to paper-and-pencil alternatives; (2) found CATSYS-produced tests clear and simple to take; and (3) appreciated review, skip, and self-adaptive options. Results from small-scale computer simulations (responses to 400 adative tests, 4 for each examinee) indicate that the accuracy of ability estimates produced by CATSYS is comparable or superior to that of estimates provided by the BILOG and MicroCAT programs. Two tables present study data. There are 26 references. Three appendices contain examples of main menus used in CATSYS, a summary of questionnaire responses concerning examinees' reactions to tests produced by CATSYS, and examples of CATSYS score reports. (Author/SLD)