The effects of dialing with a mobile telephone on driving behavior were studied. Subjects were asked to dial telephone numbers while driving an instrumented car. The effects of this telephone task on driving behavior were evaluated with several objective driving measures. These measures were then compared with a similar set collected while the drivers were just driving and while they were driving and adjusting the car radio. The effects of dialing were found to be equivalent to those of adjusting the car radio. Both rotary and pushbutton dialing were studied. The rotary dial telephone was mounted on the transmission hump of the car. Pushbutton dials were evaluated in several locations; on the dash, in the dash, in the visor area, and on a telephone handset. There were not large differences between the effects of the various designs on driving behavior. The differences that did occur, and the subjective evaluations of the drivers, favor a pushbutton dial, fixed in position and located close to the driver's line of vision of the roadway.