Sure. This works:
sol = NDSolve[{x'[t] == 1^-9 (10 - 180 y[t] - 342 z[t] - 50000 x[t]), y'[t] == (1.5^-6) x[t], z'[t] == (6.3^-8) x[t], x[0] == 7.5^-5,
y[0] == 0., z[0] == 0.}, {x[t], y[t], z[t]}, {t, 0, 50}]
You had typed NDsolve instead of NDSolve. The way you typed it the "function" was blue, when you type it right it goes "black".
Cheers,
Marco
