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The reaction p͑e, e 0 p͒p 0 has been studied very close to threshold DW # 4 MeV and at 4-momentum transfer Q 2 0.1 ͑GeV͞c͒ 2 . The scattered electron and the kinematically focused recoil proton were detected in coincidence using the 3-magnetic spectrometer setup at the Mainz Microtron MAMI. The transverse and longitudinal as well as the longitudinal-transverse and the transverse-transverse interference structure functions have been determined. From the data the s-wave multipoles E 01 and L 01 have been extracted. The experimental results are compared to recent calculations in chiral perturbation theory. [S0031-9007(98)05554-9] PACS numbers: 13.60. Le, 12.39.Fe, 25.30.Rw The pion is the lightest hadron and plays a decisive role in the structure of the nucleon. It is understood as a "Goldstone boson" reflecting the spontaneous symmetry breaking of the QCD vacuum. At low energies, where the color confinement makes the hadrons the only observable degrees of freedom, the Goldstone bosons provide a connection to QCD. Since the interaction of the light mass Goldstone bosons is weak at low energies, effective field theories can be formulated and observables can be calculated perturbatively in a momentum and quark mass expansion. This approach to low energy QCD is called "chiral perturbation theory" (ChPTh). It comprises two aspects which make an experimental verification interesting. In the first place, the intricate calculations of oneand two-pion-loop corrections and the range of validity of the renormalization of ChPTh requiring counter terms with constants not given in the framework of ChPTh have to be checked. Second, if from such a study a consistent picture emerges, the dynamic consequences of spontaneous symmetry breaking in QCD can be quantified.The foundation was laid by Weinberg (see [1] and references therein) and originally limited to systems containing only pions. Consecutively ChPTh was applied in detail to pp [2] and pN [3] reactions. However, due to the well known electromagnetic interaction, the photo-and electroproduction of pions promise particularly significant tests of the limits of validity and permit the determination of the low energy constants. The respective theoretical investigations have been performed only recently [4]. It was the aim of the experiment described in this paper to show that pion electroproduction can achieve the accuracy to provide a meaningful test of ChPTh. Different theoretical approaches, e.g., in Ref. [5] are not discussed because of space limitations.About 25 years ago, the reaction e 1 p ! e 0 1 p 1 p 0 had been investigated experimentally at threshold and in the energy region of the D͑1232͒ resonance [6-9]. But only total cross sections significantly above the pion production threshold and at relatively high 4-momentum transfers could be determined. The small cross sections close to threshold could not be measured at the old low duty cycle ͑#2%͒ electron accelerators, which also had a disturbing beam halo.The advent of high current, high duty cycle accelerators afte...

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