This paper describes experiments evaluating and optimizing an automatic gain control system, called dual front-end AGC (abbreviated as D), intended for use in hearing aids. This system has two purposes: (1) to compensate for variations in the overall level of speech from one situation to another by slowly changing its gain; (2) to protect the user from sudden intense transients without affecting the long-term gain. This is achieved by using two control voltages to determine the gain. One changes slowly as the input varies in level. Normally this component determines the overall gain. The other comes into operation when an intense transient occurs. It acts rapidly to reduce the gain, avoiding over-amplification of the transient, but its action ceases quickly after the end of the transient. We describe four experiments measuring speech intelligibility for subjects with cochlear hearing loss in which we determine optimum values for two of the time constants of the D system, namely the recovery time of the fast component and the attack time of the slow component. The experiments also compare the D system with linear amplification (L) and 'adaptive compression' (A). The results show: (1) for the D system, optimum values are about 80-150 ms for the recovery time of the fast component and 150-325 ms for the attack time of the slow component; (2) in situations where intense transient sounds are present, and there is either no background sound (experiment 1) or continuous speech-shaped noise as a background (experiment 2), the D system gives significantly better performance than the L or A systems. When the background noise is a single voice, reversed in time (experiment 3), the D and L systems give similar performance, and both are markedly superior to the A system; (3) when the level of speech is varied over a range of 30 dB (experiment 4), both D and A systems allow good performance over the whole range of levels. Performance for the L system worsens markedly at the lower levels.