Single fibres from the anterior tibialis muscles of frogs were used in paired difference experiments to investigate the long-term effect (up to 3.8 s) of tetanic stimulation on fibre stiffness during relaxation. The fibres were stretched from sarcomere lengths of 2.5 microns to 3.0 microns at constant velocities for periods ranging from 0.5 to 1.75 s. The first stretch of each pair took place when the fibres had not experienced a tetanus for at least 5 min. The second stretch took place 20-30 s after the first, but it was preceded by a tetanus (100 Hz stimulation applied for 190 ms; temperature, 23 degrees C). The force produced by the first stretch was subtracted from the force produced by the second to produce a paired difference. The fibres were held at the sarcomere length of 3.0 microns except for a brief period of time immediately prior to the stretches (1200 or 1450 ms). During those periods the fibres were shortened to 2.5 microns (250 ms) and then held at 2.5 microns, regardless of whether a tetanus was elicited, for either 950 or 1200 ms. The second stretch of each pair began either at the end of, or 250 ms after, the last stimulating pulse of the tetanus. At every velocity of stretch, the force produced by the fibres during the stretches was greater when the stretches were preceded by a tetanus than when they were not, and the additional force peaked at the conclusion of the stretches. The additional force, which was produced during the stretches following the tetani, declined for the remainder of the data acquisition period (up to 3 s) following the completion of stretches; it extrapolated to zero at 7-8 s after the completion of the stretches. The magnitude of the additional force was a non-linear direct function of the rate of stretch. Thixotropy or an increased stiffness to stretch was observed in all of the fibres following periods of quiescence.