1 Recordings have been made from primary muscle spindle afferents in split dorsal root filaments of rats anaesthetized with urethane. 2 Injection of pentetrazol (PTZ, 10 mg/kg) produced elevated discharge in the afferents with intact efferents without any charge in tension or electrical activity of the muscle of origin. 3 This elevated discharge was found to contain elements of both gamma d and gamma s activation. 4 Trimethadione (100 mg/kg), itself produced a transient increase in discharge and effectively suppressed PTZ-induced discharges of the afferents for at least 2 h. 5 These results are discussed in the context of established effects on sensory and motor systems.