5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) was normally recognised as a Cl- channel inhibitor, but its specificity is in question, since an inhibitory effect against K⁺ channels has been reported. To identify the significance of the molecules structural components, NPPB and related compounds, such as 2-(3-phenylpropylamino) benzoic acid (PPAB), 5- nitro-2-heptylamino benzoic acid (HANB) and 2-nitro-5-heptylamino benzoic acid (HANB-2) were synthesised by reductive amination using various aldehydes and amines. Using internodal cells of the giant green Characean algae, Nitella hookeri, the effects of NPPB and related compounds on cytoplasmic streaming and turgor regulation were determined. Previous experiments stated that cytoplasmic streaming was sensitive to NPPB, PPAB and HANB with IC₅₀ values of 24µmol/L, 455µmol/L, and 6.4mmol/L, respectively. In this report, the IC₅₀ values of purchased NPPB and niflumic acid were found to be 88.65µmol/L and 121.82µmol/L, respectively. Although the IC₅₀ value of purchased NPPB showed a slight difference from that of synthesised NPPB, the results of the cytoplasmic streaming experiment indicated the possibility of this analysis to be a simple assay system for analysing the effects of structural modification to ion channel inhibitors on their biological activity. Moreover, NPPB and PPAB seem to stimulate regulation of turgor pressure under hyperosmotic shock, which can be explained by a blockage of K⁺ efflux during osmotic stress leading to faster recovery of turgor regulation. Additionally, the results of cytosolic free Ca²⁺ analysis using aequorin technology also suggested that the possibility of this analysis to be used as a more direct measure of the inhibitory effect, while the cytoplasmic streaming analysis is a more indirect method. The preliminary results from this research suggest the significance of the simple assay systems for analysing the effects of structural modification ion channel inhibitors, which can be used for future study regarding ion channel structures.