The degree of recurrent inhibition of tonically firing alpha- and gamma-motoneurones to triceps surae muscles was assessed in decerebrated cats by measuring the change in probability of firing caused by an antidromic volley in other motoneurone axons. 2. In nine cats 91% (thirty-one out of thirty-three) of alpha- and 54% (twenty-five out of forty-six) of gamma-motoneurones could be inhibited by antidromic volleys in alpha-motoneurone axons. 3. The degree of recurrent inhibition, expressed as the average reduction in probability of firing during the response, was typically in the range of 59-95% for alpha-motoneurones compared to 20-85% for gamma-motoneurones. 4. The duration of recurrent inhibition was 20-50 msec for alpha-motoneurones and 5-40 msec for gamma-motoneurones. The duration was dependent upon the frequency of firing of a neurone, being shorter at high frequencies than at low frequencies. When alpha- and gamma-motoneurones had similar frequencies of discharge the durations of their recurrent inhibition were comparable. 5. Raising the strength of electrical stimulation to elicit an antidromic volley in gamma- as well as alpha-motoneurone axons never produced or increased recurrent inhibition in either type of motoneurone. 6. The quantitative differences in recurrent inhibition of alpha- and gamma-motoneurones are discussed in relation to the control of firing frequency.
