The informative power of the rate of change of the fundamental frequency (RCFF), i.e., approximately the time derivative of the fundamental frequency, has been used in two different projects on the human voice quality. One project [Hammarberg et al., Acta Otolaryngol. 90, 441–452 (1980)] deals with acoustic correlates of voice disorders. It has been shown that a perceived quality, which is commonly named “roughness,” correlates with cycle to cycle variations (perturbations) in the speech waveform. The RCFF calculated over a very short time window has been used to detect those parts of the running speech which show waveform perturbations. The results suggest a correlation between the degree of “roughness” as rated by voice experts and the amount of waveform perturbation periods in the speech. In the other project [Askenfelt and Sjölin, STL/QPSR 2–3, 74–81 (1980)] an attempt was made to find a parameter reflecting voice changes associated with psychopathological depression. The average RCFF was determined from the patient's reading of a standard text during illness and after recovery respectively. Thereby, the RCFF was calculated over a time window of approximately 50 ms. The results suggest that this RCFF measure is influenced by the mental health of these patients.