Hydrostatic pressures of 0 to 100 atmos obtained with helium gas pressure, and of 1.9×103 atmos obtained by an ice expansion bomb technique are used to measure the pressure displacement of critical temperature Tc. The coefficient (∂Tc∂p) for Sn, In, Tl, and Al is -4.7+/-0.2, -4.0+/-0.2, +0.6+/-0.3, and -2.0+/-0.2 in units of 10-5 °K/atmos, respectively. The temperature dependence of R=(∂Hc∂p)T(∂Hc∂p)Tc above 1°K is described by R=0.61+0.029T2 for Sn, and R=0.77+0.020T2 for In. This result is discussed in connection with the similarity principle, H0Tc=constant. From an analysis combining stress and isotope effects, it is seen that Tc, or Hc, is more sensitive to volume changes, holding zero-point lattice vibration amplitude fixed, than to zero-point amplitude changes, holding V fixed: (∂Hc∂lnV)q2=5.5×(∂Hc∂lnq2)V for Sn. Zero-pressure critical field data are also presented for the subject metals.