1. Isometric twitch responses of single motor units in snake costocutaneous muscle have been recorded before and after conditioning tetanic stimulation. 2. Most units showed post-tetanic potentiation of twitch tension, associated with increased maximal rate of rise of twitch tension, and in some cases also associated with prolongation of twitch contraction time. A few units showed a short phase of actual depression of the post-tetanic twich responses, followed by potentiation. 3. The time course and magnitude of post-tetanic changes of twitch tension and maximal rate of rise of tension could be described by the sum of three processes which are assumed to be maximal close to the end of the conditioning tetanus: (i) a rapidly declining potentiation (called here early potentiation, which lasted less than 2 sec) which may have a purely mechanical origin; (ii) a much slower-declining potentiation (called here prolonged potentiation, which lasted up to 15 min); and (iii) a process which lasted up to 60 sec during which twitch potentiation was reduced. The latter process (called here depressed potentiation) was usually apparent as a marked trough in the plots of twitch amplitude versus time after the conditioning tetanus, and occassionally was evident as an actual transient depression of twitch amplitude after the tetanus compared with that before. 4. The effects of the prolonged potentiation and the depressed potentiation on the maximal effect of each process close to the end of the tetanus were extracted by fitting single exponential equations to different portions of the data, using a computer program. 5. Twitch potentiation associated with increased maximal rate of rise of tension seemed to be a separate phenomenon to that associated with prolongation of contraction time, seen when conditioning tetani of higher frequency and numbers of stimuli were employed. The depressed potentiation of twitch tension tended to be partly masked in cases where contraction time was prolonged, but this did not affect the depressed potentiation of maximal rate of rise of tension. 6. The post-tetanic potentiation shown by a unit was related to the contraction time of the unit, in addition to the well known relationship to the initial twitch-tetanus ratio. The depressed potentiation may correlat more closely with the initial twitch-tetanus ratio than with the unit contraction time. 7. The magnitude of maximal depressed potentiation shown by a unit may be directly correlated to that of maximal prolonged potentiation. 8. The time constant of decay for prolonged potentiation of twitch tension tended to be related inversely to unit contraction time and directly to unit size and the maximal value of prolonged potentiation oftwitch tension. The time constant of decay for prolonged potentiation of maximal rate of rise of tension tended to be related to unit size and initial twitch-tetanus ratio, and the time constant of decay for depressed potentiation of rate of rise of tension tended to be related to unit size. 9...