The effect of pressure to similar to 90 kbar has been measured on the second harmonic generation (SHG) efficiency for the monoclinic crystals 4-aminobenzophenone (ABP), 4-Br-4＇-methoxychalcone (BMC), and 2-methyl-4-nitroaniline (MNA). In ABP and BMC the SHG efficiency increases by 2-3 orders of magnitude with some dependence on orientation. The results can be explained qualitatively in terms of a decrease in Delta k, the mismatch between the coupled fields, i.e., by pressure tuning of Delta k. Powdered samples give much smaller increases in SHG efficiency but a clearer indication of first-order phase transitions, probably between point groups of the monoclinic structure. MNA crystal exhibits a very sharp increase in SHG efficiency by a factor similar to 20 at pressures below 10 kbar. At pressures above similar to 30 kbar self-absorption by the sample becomes dominant.