Since May 1991, the GALLEX experiment — installed in the Gran Sasso Underground Laboratory — is continuously monitoring the solar neutrino signal by measuring the 71 Ge production rate in a target of 30.3 tons of nat Ga. The measured signal 77.1 ± 8.5 (stat) +4.4 – 5.4 SNU (1 σ ) [1] is well below the Standard Solar Model (SSM) prediction: ⋍ 120–130 SNU. To exclude unknown systematic effects as responsible for the observed deficit, an experiment with a (62.5 ± 0.4) PBq 51 Cr neutrino source has been performed 2. , 3. . The ratio R=0.97 ± 0.11 between measured and expected Cr-produced 71 Ge demonstrates that the deficit cannot be ascribed to unknown experimental systematics. Although astrophysical reasons could still remain as a possible explanation of the “solar neutrino puzzle”, we recall that the GALLEX result, when considered together with the results of the other solar neutrino experiments, would allow to strongly constrain the neutrino masses and mixing angles in the framework of the MSW effect.