Monkeys can gradually change the amplitude of the wholly segmental, largely monosynaptic, spinal stretch reflex (SSR) when confronted by a task requiring such change (15-19). Change develops over months and may reverse and redevelop at similarly slow rates. We investigated the persistence of SSR amplitude change over nonperformance periods of up to 38 days. Eight animals with chronic EMG electrodes learned to maintain elbow angle and a given level of biceps background EMG against constant extension torque. At random times, a brief additional extension torque pulse elicited the biceps SSR. In the control mode, reward always followed. Under the SSR increase or SSR decrease mode, reward occurred only if the absolute value of biceps EMG in the SSR interval was above or below a set value. Animals completed 3,000-6,000 trials/day over data-collection periods of 2-17 mo. Animals worked first under the control mode for up to 60 days and then under the SSR increase or SSR decrease mode for up to 274 days. Mode was switched once or twice more (SSR increase to SSR decrease or vice versa) over subsequent months. Animals responded to each SSR increase or SSR decrease mode exposure with gradual mode-appropriate change in SSR amplitude. Mode exposures were interrupted by gaps in performance of 10-38 days. Gaps produced transient 10- to 15% decreases in SSR amplitude under the control mode. This nonspecific decrease disappeared over the first week of postgap performance. Under the control mode, gaps had no other effects on SSR amplitude.(ABSTRACT TRUNCATED AT 250 WORDS)