Laboratory techniques are now available for measurement of both male and female genital engorgement during sexual arousal. When laboratory subjects are given information on their rate and amplitude of tumescence (biofeedback), they appear to acquire greater voluntary control of the response. Data from two experiments and a case study are presented to illustrate the application of biofeedback in both facilitation and suppression of penile responding. A PhD student is currently conducting analogous research with female subjects. Clinicians are cautioned against careless application of biofeedback techniques for the treatment of erectile inadequacies.