A hand-held, battery-powered synthetic speech aid for the non-vocally disabled has been constructed. The device accepts as its input, largely unrestricted text keyed by the user. This is converted by text-to-speech software, based on 349 letter-to-sound rules and some simple rules of continuity, intonation and stress, to appropriate control signals which drive a single-chip (series formant) speech synthesizer. A number of implementation constraints are imposed by portability; the system has, as far as possible, been designed using CMOS components. To extend the time for which the system will operate between battery charges, power saving facilities are incorporated. Hand-held use implies the need for a one-handed keyboard: a unique integral keyboard is used, designed to minimize the visual search time to locate a letter key. Considerable attention has been paid to rule-search strategies, the handling of 'exceptions' which violate the letter-to-sound principle and the resolution of conflicts when more than one rule might apply. The quality and intelligibility of speech from a rule-based system is typically poor, and every effort has been made to improve it. Limits on possible improvement are, however, set by the use of a proprietary single chip synthesizer and by the minimal nature of a portable system. To facilitate the task of composing messages, a two-line liquid crystal display is provided together with a range of editing functions. The display can also be shown to the message receiver should he/she be deaf, or used for silent communication as an analogue to 'whispering'.(ABSTRACT TRUNCATED AT 250 WORDS)