The just-noticeable difference (JND) for selected aspects of voice fundamental frequency (F0) contours was determined by varying the F0 control parameter of a digitally simulated terminal analog speech synthesizer. Data were obtained from three subjects for a number of 250-msec segments of the synthetic vowel /ɛ/ differing only in fundamental frequency. Results indicate that the subjects can detect a change of 0.3 Hz in a constant F0 contour when F0 = 120 Hz, but the JND is an order of magnitude larger (2.0 Hz) when the F0 contour is a linear descending ramp (32 Hz/sec). Sensitivity to rate of change of F0 in linear ramps is surprisingly good; greatest sensitivity occurs when one ramp increases and the other decreases (JND = 12 Hz/sec). High-pass filtering of the stimuli improves performance slightly, suggesting that the fundamental component is not involved in the detection of changes in F0. Substitution of the synthetic stimulus /ya/ with its dynamic formant contours in place of /ɛ/ degrades performance only very slightly. Implications of these data for models of pitch perception mechanisms are discussed.