The non-synaptic membrane currents of muscle fibres have been studied in late embryogenesis of Drosophila melanogaster using the voltage-clamp technique in wild-type and Shaker mutant third instar larvae. Five currents were found in the wild type muscle membrane at this embryonic stage: one fast inward Ca current (ICa), two fast outward K currents (IA and IAcd) and two slow outward K currents (IK and IC). IAcd and IC are Ca-dependent. Several procedures were used to separate IAcd from IA: IAcd is present in Shaker mutants which are characterized by the absence of IA (Salkoff and Wyman 1981); IAcd, but not IA, is suppressed by Co2+ (10 mM) or La3+ (1 mM); IAcd shows steady-state inactivation at more positive potentials than IA; IAcd, unlike IA, is 3,4-diaminopyridine (3,4-DAP) resistant. Furthermore, tetraethylammonium (TEA, 20 mM) which is known to be uneffective on IA, blocks IAcd. IAcd could not be triggered by using strontium or barium as calcium substitutes. By partial substitution of Ca by Ba or Sr ions, it was found that Ba, but not Sr, blocks the IAcd channel. A non-inactivating, TEA sensitive, Ca-dependent K current (IC), which gave N-shaped I-V plots, could be separated from IK by using Ca-channel blockers. IC and IK activate at membrane potentials of about -25 mV and -10 mV, respectively. The participation of IAcd and IC to membrane electrophysiology is discussed.