Needs["Graphics`PlotField`"]; a = 0.04; b = 0.003; c = 0.0015; i0 = 0; f = Expand[v (a - v) (v - 1) - b/c v + i0]; Solve[f == 0]; bild1 = Plot[{b/c v, v (a - v) (v - 1) + i0}, {v, -0.3, 1}, PlotRange -> {-0.1, 0.2}, ImageSize -> {400, 300}, DisplayFunction -> Identity, AxesLabel -> {"u", "v"}]; bild2 = PlotVectorField[{v (a - v) (v - 1) - w + i0, b v - c w}, {v, -0.25, 1, 0.2}, {w, -0.1, 0.2, 0.04}, ScaleFactor -> 0.15, DisplayFunction -> Identity]; bild = Show[Evaluate[{bild1, bild2}], DisplayFunction -> $DisplayFunction, ImageSize -> {400, 300}];