Background: Residue 436 within 5-HT3A cytoplasmic portals determines single channel conductance (?) and rectification. Results: M3-M4 loop truncation to 75 residues spared inward rectification; truncation to 70 abolished rectification and increased -. Human Cys-loop neurotransmitter receptor M3-M4 loops all exceed 70 residues. Conclusion: M3-M4 lengths of >70 are critical. Significance: Cytoplasmic portals may be essential for normal function of Cys-loop neurotransmitter receptors.