Heterodyne detection (mixing) at millimeter wavelengths has been demonstrated in a distributed structure of bulk semiconductor, combining large dimensions, ease of matching, etc. The nonlinear mechanism utilized involves the increase in conductivity of indium antimonide at cryogenic temperatures due to the heating of the free electrons by absorption of RF power. The conversion loss and noise properties were analyzed. A minimum conversion loss of 6 dB is calculated. The method of operation was verified by constructing and testing a low-frequency prototype operating at one Mc/s which uses a commercially available room-temperature bolometer. The measured conversion loss of 25.0 dB checked the analysis within 2.2 dB. Millimeter mixing in the 8-mm region (30 to 40 Gc/s) was demonstrated using bulk n-type indium antimonide as the nonlinear element. Conversion losses as low as 18 dB were measured at 4.2°K, using a 1.5 Mc/s IF, from which a conversion loss as low as 12 dB was calculated by correcting for dissipative loss and other effects. For low conversion losses, an upper limit is placed on the IF frequency by the time constant of the InSb mixer element, which is near 10 -7 second. Envelope detection measurements were carried out at 8 mm and 2 mm. The use of compensated InSb elements at liquid-helium temperature has yielded responsivities as high as 3500 V/W at 35 Gc/s and 150 Gc/s.