1. Isolated and glycogen-depleted motor units (MUs) have been studied in normal and reinnervated tibialis anterior (TA) muscles of the rat to examine 1) the correspondence between physiological and histochemical classifications, 2) the extent to which unit properties cluster according to type, 3) the relation between unit force and fatigability, and 4) the extent to which reinnervated MUs recover their former properties. 2. MUs were isolated by ventral root dissection and stimulation in reinnervated and normal TA muscles, 3.5-8 mo after common peroneal (CP) nerve section and resuture and in age-matched control rats, respectively. The units were characterized physiologically for classification into four types: slow twitch (S), fast twitch, fatigue resistant (FR), fast twitch fatigue intermediate (FI), and fast twitch fatigue sensitive (FF). Four muscle fiber types were identified histochemically with the use of a modification of the techniques of Brooke and Kaiser, and Guth and Samaha to delineate fiber subtypes on the basis of the pH sensitivity of myofibrillar adenosine triphosphatase (ATPase). 3. Neither the time-to-peak twitch force development nor the profile of unfused tetanus ("sag test") was unambiguous in separating fast from slow MUs. However, all units with a time to peak greater than 22 ms were fatigue resistant, and this time was chosen to delineate fast from slow. The fast unit population was further subdivided on the basis of their fatigability. There is normally a small proportion of S units (6% S) that increased to 20% after reinnervation. Although the fast population was subdivided, there was a continuous distribution of fatigue indexes in normal and reinnervated muscles with the highest number of fast units falling into the FI category. The proportions of fast units were 28% FR, 45% FI, and 21% FF in normal muscles and 29% FR, 38% FI, and 13% FF in reinnervated muscles. 4. In normal muscles, delineation of fast and slow fibers and subdivision of fast fiber types on the basis of acid and alkali stability of myofibrillar ATPase provided a histochemical classification that showed 78% correspondence with physiological classification of the same identified units. In reinnervated muscles the correspondence between physiological and histochemical classifications was reduced to 72%. 5. The normal correlation between MU fatigability and isometric force in TA muscles was not seen in reinnervated muscles that contained more FR MUs. Mean fatigue index from normal units was significantly less at 0.55 +/- 0.03 (mean +/- SE) compared with 0.68 +/- 0.03 from reinnervated units.(ABSTRACT TRUNCATED AT 400 WORDS)