During growth of identified crayfish muscle fibres from a diameter of 20 to 400 micron, the excitatory junctional potential (e.j.p.) amplitude was found to be independent of diameter. Thus, e.j.p. amplitude was maintained during growth in spite of a 21-fold decrease in miniature excitatory junctional potential (m.e.j.p.) amplitude previously reported (Lnenicka & Mellon, 1983). The maintenance of e.j.p. amplitude was found to be partially due to a 5-fold increase in quantal release at 'active sites' during growth. In order to determine whether the increase in transmitter release can be regulated by the rate of muscle fibre growth, the rate of growth was experimentally reduced. By decreasing the resting length of the muscle during growth, the rate of increase in the diameter was reduced by approximately 50% compared with the contralateral control muscle fibres. The input resistance and the m.e.j.p. were appropriately larger in the smaller-diameter experimental fibres. However, e.j.p. amplitude in the experimental fibres was not significantly different from that in the contralateral control fibres. This was apparently due to the significantly smaller quantal release at active sites on the experimental fibres compared with control fibres. Thus, experimental alteration of the rate of muscle fibre growth results in regulation of transmitter release, suggesting that the muscle fibre may control the increase in transmitter release seen during normal growth.