A study of switching effects in vapour-grown zinc sulphide platelets (1-100 μm thickness) contacted with silver or indium is described. Depending on temperature, both `monostable' and `bistable' switching was observed. In the `bistable' region crystals exhibited a variety of metastable conduction states; these are discussed in terms of filamentary behaviour and a `variable donor density' model in the accompanying paper (II). This paper attempts to develop an overall model of the switching by concentrating on transient behaviour. Some attempt is made to describe the type of centre necessary to explain the observed properties.