The muscle fibre electrical properties, miniature excitatory junctional current (m.e.j.c.) and miniature excitatory junctional potential (m.e.j.p.) were studied during growth of an identified crayfish muscle fibre from a diameter of 20 to 400 microns. The specific membrane resistance (Rm), and the specific internal resistance (Ri), of the muscle fibre were independent of fibre diameter (d) during growth. The current-voltage relation has a similar shape in large and small fibres, indicating that voltage dependence of Rm does not change during growth. The input resistance (R0) was approximately proportional to d-1.5, as predicted theoretically. The specific membrane capacitance (Cm) and the membrane time constant (Tm) increased linearly with fibre diameter, apparently as a result of the contribution of the tubular capacitance to Cm. The decrease in R0 and the increase in Tm should have resulted in a 90-fold decrease in m.e.j.p. amplitude during growth of the fibre from a diameter of 20 to 240 microns. However, m.e.j.p. amplitude was found to decrease only 21-fold. This discrepancy was shown to result from an increase in m.e.j.c. amplitude and duration during growth. There was 2.9-fold increase in m.e.j.c. amplitude and a 2.7-fold increase in m.e.j.c. duration over the range of muscle fibre growth studied. This increase in the m.e.j.c. apparently results from an increase in the magnitude and duration of the synaptic conductance change produced by a quantum of transmitter. Throughout the range of muscle fibre diameters studied, the muscle fibre effective input impedance for the m.e.j.c. was 17-19% of R0. This is due to the relatively large Cm and the short duration of the m.e.j.c.