The properties of acetylcholine (ACh) receptors of the singly innervated posterior latissimus dorsi (PLD) and the multiply innervated anterior latissimus dorsi (ALD) muscles of the chicken were investigated. Studies were made on chicks from 17 days in ovo to 14 weeks after hatching. Focal extracellular recordings and intracellular recordings in voltage clamped fibres were made. 2. Peak amplitudes of miniature end-plate currents (m.e.p.c.s) of the two muscles were not significantly different. The time constants of decay (tau D) were similar in both muscles, although tau D in the PLD was generally smaller than in the ALD (usually by less than 25%). M.e.p.c. decays in both muscles were well described by a single exponential. 3. The conductance (gamma) and average lifetime (tau N) of end-plate channels activated by ionophoretically applied ACh were calculated from records of current fluctuations. Noise spectra were well fitted by a single Lorentzian function. Values obtained in PLD did not differ significantly from those obtained in the ALD. There was not difference in the ACh null potential. 4. The voltage and temperature sensitivities of the ACh-activated channels in both muscle types were very similar. 5. With age there was a slight decrease in tau D: from about 6 to 5 msec in the PLD and from about 7 to 5 msec in the ALD (at -40 mV). The change in tau N with age was even less marked. However, during development, gamma almost doubled in both muscles, increasing from about 20 to 35 pS. 6. The results provide no evidence for the hypothesis that the different pattern of innervation causes marked differences in the ACh-activated channels of singly and multiply innervated muscles.
