Pelvic muscle contractions during sexual response can be monitored conveniently by the anal probe method described. Eleven young adult male subjects were each recorded for three sessions of masturbation to orgasm. Electrical signals from an anal pressure probe were automatically digitized by computer. Orgasmic contractions were easily distinguished from voluntary contractions by the steadily increasing intervals and complete muscle relaxation between orgasmic contractions. At orgasm each subject produced a characteristic series of contractions starting abruptly at an intercontraction interval of about 0.6 seconds, and continued for 10 to 15 contractions at an increasing increment of about 0.1 second per contraction. Pressure amplitude, representing the force of contractions, increased from the beginning of the regular series to a maximum at the seventh or eighth contraction. Area under the pressure curve, reflecting muscular exertion during contraction, generally increased throughout the regular series. Each man's pattern of contractions was very similar from one session to the next and distinguished his records from others'. Individuals' patterns could be grouped into three types, based chiefly on the location of the regular contraction series within the subjective span of orgasm. The most common type was a simple series of regular contractions. It had the shortest duration and fewest contractions. The next most common pattern began with the regular series, followed by a number of irregular contractions. This type was longest in duration. One man with a third type of intermediate duration, had a number of preliminary contractions before the series of regular contractions began in midorgasm.