The German Constitutional Court is considering whether the proposed EU permanent bailout facility, the European Stability Mechanism, is compatible with national law.
German Finance Minister Wolfgang Schaeuble has warned the judges that any significant delay in Berlin approving the ESM could fuel financial market turbulence.
However, opponents of the ESM argue it, and the fiscal treaty, diminish the constitutional power parliament has over the national budget.
The European Stability Mechanism is one of the key bulwarks established by eurozone leaders to end bond market volatility during the eurozone crisis.
The €500bn fund can provide bailouts to eurozone countries in fiscal trouble, with conditions attached.
The ESM is being ratified by various eurozone countries. It was passed by a large majority in the German lower house, the Bundestag, last Friday.
Yet opponents argue that joining the bailout fund would impose limits on the German parliament's constitutional power to say how taxpayer money is spent. They are seeking a court order preventing it from being signed into law, pending a full hearing.
However, the eight judges hearing the case in Karlsruhe were told by Mr Schaeuble that the ESM was an important step towards a European stability union and would send clear signals that solidity and solidarity belong together.
The Federal Constitutional Court usually is supportive of greater EU integration but has demanded in the past that the government have greater consultation with the parliament.
A ruling on the case will not be delivered today.