The paper describes circuits for performing logical operations in data processing. They operate reliably at 1 Mc/s and at a maximum ambient temperature of 45° C. The logic circuits use an alloy-junction transistor with an alpha cut-off frequency exceeding 10 Mc/s and an aluminium-bonded diode. Complete digital systems can be built using a number of logic units called ‘bricks’, in conjunction with the circulating pulse stores described in a companion paper [Showell, H. A., Barrow, C. W. M., and Collis, R. E. (Convention paper*)]. A set of simple rules determine the interconnections that can be used between units.The logic units described are gates, buffer amplifiers, inhibit-inverter amplifiers, bistable triggers and monostable triggers. D.C. coupling is used between units in order to obtain uniform operation irrespective of the time pattern of the pulse signals. Each one-microsecond digit period is terminated by a common discharge pulse of short duration that restores all transistors, by removing minority carriers from the base regions. No regenerating amplifiers are used, because the circulating pulse stores perform the dual functions of storage and of regenerating pulses, which have suffered attenuation and delay in the logic circuits.Each circuit unit is on a small printed wiring card. These are assembled in the required combinations on larger cards which form convenient plug-in packages.