Facilitation of transmitter secretion from motor nerve terminals following one or more conditioning stimuli was examined in mouse sternomastoid muscles. Following a single conditioning stimulus at 20 degrees C, facilitation decayed exponentially during the first 70-80 msec with a time constant of 60 msec. After 70--80 msec, a small slower component of facilitation was apparent. Initial facilitation (obtained by extrapolation back to 'zero' time) had a value of approximately 0.5. Following more than one conditioning stimulus (2-6), initial facilitation was greater but the pattern of decay was similar, the slower component becoming more obvious as the number of conditioning stimuli was increased. The slow decay phase also appeared exponential. The pattern of decay of facilitation could be well fitted by the sum of two exponentials, F1 (0)exp(--t/tau 1) + F2(0)exp(--t/tau 2). After a single stimulus at 20 degrees C, tau 1 and tau 2 had mean values of 35 and 163 msec. The main effect of increasing the number of conditioning stimuli was to increase (F1(0) and F2(0) with little change in tau 1 or tau 2. Changing temperature from 30 degrees C to 10 degrees C increased F1(0) and tau 2 but had relatively little effect on F2(0) and tau 1.