The relationships between the Solar Magnetospheric (SM) y- component of the interplanetary magnetic field (IMF) and the lobe and plasmasheet magnetic fields have been studied for the two ISEE-3 deep tail passes. It is found that for positive sector IMFs. 13% of the interplanetary magnetic field penetrates into the aberrated north-dawn and south-dusk lobe quadrants, and about the same amount in the north-dusk and south-dawn lobes quadrants for negative sector IMFs. For the above cases, field penetration is significantly less for opposite polarity IMFs. The former results are generally consistent with open magnetospheric models, but the latter (the lack of response in certain quadrantsa) are unexplained by theory at this time. If the magnitude of the plasmsheet By fields are related to plasma pressure anisotropies, very small anisotropies of P⊥/P∥≈ 1.01 are expected.