the gunn effect as a function of pressure has been studied in n~type insb. the gunn threshold field is found to decrease with increasing pressure reaching a minimum at p ~ 9 kbars. it then increases while the oscillations weaken with increasing pressure up to 12 kbars. low field hall and conductivity measurements have also been made in an attempt to obtain a possible explanation for the threshold minimum. it is observed that the impurity level moves away from the conduction band approximately exponentially with increasing pressure. by 9 kbars the donor ionization energy is large enough that considerable freeze-out or deionization of donor states occurs. the reduction in carrier number is unfavourable to the gunn process and hence leads to an increase in the threshold field for pressures above 9 kbars.