You've got `Evaluate` in the wrong place. It has to be of the form Plot[Evaluate[stuff to plot...],...] So this seems to work: Manipulate[ Plot[Evaluate@{ (ReplaceAll[Paorta[t], NDSolve[{Paorta'[t] == 1/Caorta ((1/2*k*(1 + Cos[ω t]) + 10 - Paorta[t])/ Piecewise[{{Ro, 1/2*k*(1 + Cos[ω t]) + 10 - Paorta[t] > 0}}, x*Ro] - Paorta[t]/Rsystemic), Paorta[0] == 90}, {Paorta[t]}, {t, 0, 10}]]), (1/2*k*(1 + Cos[ω t]) + 10), (((1/2*k*(1 + Cos[ω t]) + 10) - ReplaceAll[Pao[t], NDSolve[{Pao'[t] == 1/Caorta ((1/2*k*(1 + Cos[ω t]) + 10 - Pao[t])/ Piecewise[{{Ro, 1/2*k*(1 + Cos[ω t]) + 10 - Pao[t] > 0}}, x*Ro] - Pao[t]/Rsystemic), Pao[0] == 90}, {Pao[t]}, {t, 0, 10}]])/Ro)}, {t, 0, 10}, ImageSize -> Large, PlotRange -> Full, PlotLegends -> {"Aortic Pressure", "Pressure in Left Ventricle", "Flow"}], {{Caorta, 1/.48}, 1, 6}, {{Rsystemic, 3.1}, .1, 6}, {{x, 8000}, 1, 10000}, {{ω, 2 π}, π, 3 π}, {{k, 110}, 60, 200}, {{Ro, .01}, .007, .05}] `Plot` has the attribute [`HoldAll`](http://reference.wolfram.com/language/ref/HoldAll.html). [`Evaluate`](http://reference.wolfram.com/language/ref/Evaluate.html) overrides holding an argument exactly when it is the `Head` of the argument. That's why the placement is important.