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Isolated single fibres from the anterior (a.l.d.) and the posterior (p.l.d.) lattissimus dorsi muscles of embryonic and young chicks were used to study in vivo development of membrane electrical properties. Isolated fibres were obtained by an enzymatic dissociation procedure. Intracellular micro-electrode recordings from isolated fibres and from fibres in intact muscles showed that the dissociation procedure did not significantly alter resting membrane potentials, input resistances or membrane time constants (tau m). The 14 day embryonic fibres of a.l.d. and p.l.d. did not have a measurable resting conductance to Cl-. At hatching, about 70% of the resting conductance in p.l.d. fibres was due to Cl-. Membrane electrical properties were estimated from the analysis of voltage responses to intracellular injection of rectangular pulses of current. At 14 days in ovo, membrane resistance (Rm) was approximately 20 k omega cm2 and membrane capacitance (Cm) was 1-2 microF/cm2 for both a.l.d. and p.l.d. The mean membrane length constants (lambda) were 1.7 mm for a.l.d. and 1.5 mm for p.l.d. For p.l.d., the values of Rm, tau m and lambda decreased as development proceeded. For a.l.d., there was no change in these values by the time of hatching (21 days). The decreases in the electrical constants for p.l.d. fibres were partly explained by the appearance of a resting Cl- conductance during the last week of embryonic development.