Single metastable nanopores, appearing before the actual electroporation under constant-current conditions, are used to characterize the onset of electroporation. Unlike the long-lived electropores typical of the current controlled methods, these pores survive for milliseconds and observing them is possible due to slow development of electroporation, provided by the gradual accumulation of charges on a planar membrane. Analysis of the metastable pore appearance frequency and lifetime shows the first introductory stage of electroporation. During this stage two species of metastable pores open, the majority of very low conductance that seem not fully developed as hydrophilic electropores. The experiments reveal that voltage value defines the electroporation onset while the current value affects the rate of electroporation. Membrane capacitance has a great impact on the membrane susceptibility to the pore appearance, related to its thickness and integrity. Pores of nonperfect membranes appear more easily, but they do not live any longer than others.