Because of their minimal cost, size, and weight, handheld personal digital assistants (PDAs) are appealing as a means for administering response time tasks "in the field" or to participants in longitudinal studies who need repeated access to the testing equipment. We have developed a software package that allows investigators to author simple scripts on their desktop computers and administer the compiled tasks on PDAs. MiniCog presents instructions, practice trials with auditory feedback, and visual stimuli in random or fixed sequential orders with fixed or variable durations and intertrial intervals and records single-button responses and response times. Data from each trial are stored with a user ID, user-entered demographics and notes, and a time-and-date stamp, for later upload to a desktop computer. MiniCog performs automatic calculations of mean response time and error rate, available to users with a password and stored with the other data for upload.