The cat gastrocnemius muscles of one hind leg were stretched sinusoidally with amplitudes between 10 micrometer and 2.5 mm and frequencies between 1 and 30 Hz. The stretch response of deefferented muscle spindle afferents and of Ia-activated cells within Clarke's column were investigated by means of extra-cellular recordings of action potentials. The responses to 20-50 cycles were displayed in impulse patterns (raster diagrams) of the responding action potentials. The impulse patterns of Ia afferents exhibited a high degree of phase-locking (regularity) on the stretch cycle of amplitudes of about 50 micrometers at 3 Hz and all higher amplitudes or frequencies. At comparable stretch parameters the regularity in Ia afferents was 4-6 times larger than in Ia-activated DSCT neurons. The regularity in the DSCT patterns increased with an increase in stretch frequency. The impulse patterns of DSCT cells exhibited a high negative correlation between successive interspike intervals (-0.4 to -0.6) at low stretch frequencies (less than 3 Hz), which decreased with an increase in stretch frequency.