Maximal twitch contractions of fast muscles of the cat caused an increase in phosphate concentration in the venous plasma from them: this efflux was greater as the contraction frequency was increased. At any given frequency of contraction, the phosphate efflux from contracting gastrocnemius was less than that from tibialis anterior and extersor digitorum longus, which have a higher proportion of fast fibres and exhibit greater functional hyperaemia. Soleus muscles, when contracting, released hardly any additional phosphate, except in the one experiment in which the muscle exhibited a functional hyperaemia. There was thus a consistent relationship between the extent of functional hyperaemia and phosphate efflux in different muscles and within any one group of muscles. Inorganic phosphate, given close arterially as NaH2PO4, was shown to be vasodilator.NaH2PO4 was much more potent than Na2HPO4, though this did not seem due simply to the associated change of pH. The functional hyperaemia of fast muscles could be matched, qualitatively and quantitatively, by injections or infusions of NaH2PO4. The possibility is discussed that the contraction hyperaemia of fast muscles is functionally related to phosphate release into the interstitial fluid during contractions.