The current density (J) is a parameter routinely used to characterize individual ionic membrane currents. Its evaluation is based on the presumption that the magnitude of whole-cell ionic membrane current (I) is directly proportional to the cell membrane capacitance (C), i.e. I positively and strongly correlates with C and the regression line describing I-C relation intersects the y-axis close to the origin of coordinates. We aimed to prove the presumption in several examples and find whether the conversion of I to J could be always beneficial. I-C relation was analysed in several potassium currents, measured in rat atrial myocytes (in inward rectifier currents, I-K1, and both the constitutively active and acetylcholine-induced components of acetylcholine-sensitive current, I-K(Ach)CONST and I-K(Ach)ACH), and in rat ventricular myocytes (transient outward current I-to). I-C correlation was estimated by the Pearson coefficient (r). A coefficient (k) was newly suggested describing deviation of the regression intercept from zero in currents with considerable r value. Based on mathematical simulations, I was satisfactorily proportional to C when r >= 0.6 and k <= 0.2 which was fulfilled in I-K1 and I-K(Ach)ACH (r = 0.84, k = 0.20, and r = 0.61, k = 0.06, respectively). IeC correlation was significantly positive, but weak in I-K(Ach)CONST (r = 0.42), and virtually missing in Ito (r = 0.04). The impaired I-C proportionality in I-K(Ach)CONST and Ito likely reflects heterogeneity of the channel expression. We conclude that the conversion of I to J should be avoided when I-C proportionality is absent. Otherwise, serious misinterpretation of data may arise. (C) 2019 Elsevier Ltd. All rights reserved.