Nonlinear oscillations of proto-giant-planets have been found in recent numerical calculations relevant to planetary formation. Pulsations are excited in two phases of the protoplanetary evolution, (a) In an ‘instability strip’ at core masses of typically 0.2M⊙ (M⊕ is the earth mass). Perturbations grow into the nonlinear domain and saturate into perodic variations with relative luminosity-amplitudes of 0.2m (b) At the so called critical mass (typically at Mcore ≈ 15M⊕). There the pulsations drive a strong mass loss. A large portion of the envelope is ejected. Then the mass loss fades and the envelope settles into a new quasi-equilibrium. This remnant — a post nucleated instability protoplanet — has a compact envelope and is in core and envelope mass similar to Uranus and Neptune.