The intrafusal muscle fiber(s) activated in cat peroneus tertius spindles by single static gamma (gamma s) axons were identified by exclusively physiological criteria based on the different contractile properties of chain and bag2 fibers. 2. The identification rested both on the features of primary ending discharges observed during gamma s electrical stimulation at a rate of 30 pulses per second (stimulation at 30/s) and on cross-correlograms constructed during stimulation at 100/s. Three types of primary ending activation could be distinguished. 3. Type F (fast) activations are characterized, at 30/s, by either a 1-to-1 driving or a very irregular increase in firing arising from a level close to the frequency of stimulation and by the presence in cross-correlograms of significant peaks. They are ascribed to chain fibers whose contractions, at 30/s, present large oscillations and, at 100/s, are still incompletely fused. 4. Type S (slow) activations are characterized, at 30/s, by a sustained and generally regular increase in firing and by the absence of significant peaks in cross-correlograms constructed during stimulation at 100/s. They are ascribed to bag2 fibers whose contractions are nearly fused at 30/s and completely fused beyond 60-70/s. 5. Type M (mixed) activations are characterized, at 30/s, by an irregular increase of discharge above a level distinctly higher than the frequency of stimulation and by the presence of significant peaks in cross-correlograms. They are ascribed to the coactivation of chain and bag2 fibers for two reasons: first, they have some features of both type F and type S activations; and second, they are readily reproduced by stimulating together two axons supplying the same spindle, one exerting a type F activation, the other a type S activation. 6. In seven experiments the distribution of 42 single gamma s axons was determined by observing the type of activation they exerted on several spindles (from 3 to 6). Thirty-five axons (83%) were classified "nonspecific" because the type of activation (F, S, or M) varied from one spindle to the other. Seven axons (17%) were classified "specific" because the type of activation was the same in all spindles: either type F for five axons (12%) or type S for two axons (5%). A statistical analysis of the distribution of all activations showed that the proportions of specific axons were not significantly different from those predicted by chance.
