Résumé : Alkaline phosphatase (AP) activities are present in rat gastrocnemius:48.7, plantaris: 68.9, tibialis anterior: 69.1 and soleus: 96.7 nmol phenol. min-1. 100 mg muscles-1. These concentrations are one and two orders of magnitude lower than those observed in duodenum and placenta, but similar to those observed in liver. Response to activators/inhibitors and electrophoretic behaviour assign the muscle AP activities to the rat liver/placenta isoenzyme group. Motor denervation does not affect significantly the total muscle AP content within the first 30 postoperative days, however the concomitant variations in muscle weight are responsible for wide differences in AP concentrations between innervated, denervated and reinnervated muscles. Parallel determinations of radiophosphate uptake and AP activities failed to document a necessary link between the two variables, i.e. between enzyme concentration and phosphate ion transport.