Combined a Fitts's law tapping-task with a serial digit-naming task having 4 levels of stimulus-response complexity in a study with 14 undergraduates. Baseline data were also established for tapping and naming tasks performed singly. In simultaneous-task performance, an increase from 3.9 to 10 bits/sec required by the tapping task was accompanied by a decrease in performance on the naming task. When single- and simultaneous-task performance were both analyzed, an interaction between tapping and naming was obtained, supporting a limited-capacity channel. However, when only simultaneous-task performance was evaluated, effects of tapping and naming were additive, supporting a stage model of information processing. Results stress the processing demands of a response execution and control stage. (19 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)