TTRPM4 and TRPM5 are structurally related cation channels (40% identity) forming a subgroup within the melastatin (M) family of “transient receptor potential” (TRP) channels. Both channels are widely expressed in excitable and non-excitable cells. TRPM5 is highly expressed in taste receptor neurons, and mutations in this channel might be associated with the Beckwith-Wiedemann syndrome. We present here functional data of TRPM4 and TRPM5 channels (human and mouse) after heterologous expression in HEK293 cells. A comparison of activation properties and modulation of both channels might unravel functionally important structural determinants of these channels. Both channels require intracellular Ca²⁺ for activation, but are impermeable for Ca²⁺.Activation of TRPM4 by Ca²⁺ is transient and requires higher (µM) Ca²⁺ concentrations than for TRPM5. The Ca²⁺ dependence of TRPM5 activation is also steeper than that of TRPM4. Exposure to constantly elevated [Ca²⁺]_i induced a decrease of the currents through both channels. TRPM4 but not TRPM5 reactivates after a delay of some minutes. Reactivation is accelerated when cells were depleted of ATP under hypoxic/glucose free conditions. Deletions in the N-terminus including possible Ca²⁺/calmodulin binding sites result in non-functional channels. However, C-terminal truncations form functional channels with a substantially reduced Ca²⁺– activation, pointing to a possible Ca²⁺ sensor located in the C-terminus. TRPM4 and TRPM5 are voltage-dependent. They inactivate rapidly at negative potentials and activate more slowly at positive potentials. This voltage-dependent behaviour results in a striking outward rectification of the steady state current. Voltage dependence of activation is of the Boltzmann-type, i.e. positive potentials increase the fraction of open channels and negative potentials decrease it (Nilius et al. 2003). Activation of TRPM5 at positive potentials is faster and shifted towards more positive potential as compared with TRPM4. Voltage dependence is not due to block by divalent cations or to a voltage-dependent binding of intracellular Ca²⁺ to an activator site. A kinetic scheme for the voltage dependence of TRPM4 will be presented, which can explain kinetic and steady state properties of both channels as well as the current decrease during repetitive stimulation. Mutations of lysine/arginine residues in transmembrane helices of TRPM4, which are conserved in TRPM5, change the voltage dependence but also the concentration range for activating Ca²⁺.These residues are conserved in TRPM4,5, and 8 but not in TRPM7 and resemble the voltage sensor in Shaker potassium channels.ATP, ADP, AMP, AMP-PNP and adenosine induce a fast and reversible block of currents through TRPM4. Block by ATP is not affected by intracellular Mg²⁺.IC₅₀ for block by ADP and ATP^4-is ~2 µM. GTP, UTP and CTP do not induce a similar block. TRPM5 is insensitive to ATP. TRPM4 has three Walker B motifs and an ABC-transporter signature motif, which are not conserved in TRPM5. Mutations of the ABC motif induce a dramatic inactivation of TRPM4 currents at positive potentials, indicating that this region modifies the kinetic behaviour of TRPM4 and might explain kinetic differences between TRPM4 and TRPM5.Spermine blocks currents through TRPM4 and TRPM5 (IC₅₀ of ~60 µM). Possible aspartate and glutamate interaction sites for polyamines, flanking the pore region, are conserved in both channels.In conclusion, TRPM4 and TRPM5 are more than only Ca²⁺ activated cation channels. They are characterized by an intrinsic voltage sensing-mechanism, which may be functionally important in excitable tissues generating plateau-like or bursting action potentials. The most dramatic difference between TRPM4 and TRPM5 channels, among the several significant quantitative differences, is the sensitive block of TRPM4 channels by ATP, which does not occur in TRPM5.