The purpose of this investigation is the determination of a simple criterion of local space charge effect in conventional avalanche counters at moderate specific ionization. Parallel-plate avalanche counters (PPAC) with an electrode spacing d from 0.1 to 0.4 cm have been used to register low-energy alpha particles at n-heptane vapour pressures of p ł 5 Torr. The criterion determined is a simple h/p^m function describing the variability of the product of effective gas amplification and actual particle energy loss, where both the numerical h coefficient and the exponent m assume different values for the various values of d. The product variability at higher gas pressures in PPAC detector – having regard to practical side of criterion determined – can be relatively well described by a single linear function. The results obtained are discussed from their veracity point of view.