The relationships were studied between sensory responses and behavioural responses to the same stimulus. Sensory and behavioural reactions were both quantified according to stimulus type and concentration. Correlations between relative sensory responses and relative behavioural responses were determined to obtain some insight into processing in the central nervous system. The larvae of two phytophagous insects were studied: Pieris brassicae L. and Mamestra brassicae L. Pieris is an oligophagous species, restricted to Cruciferae, and Mamestra for reasons of comparison is a polyphagous insect. Phytophagous larvae have three types of gustatory sensilla (present in symmetric pairs), i.e. two pairs (a lateral and a medial) of styloconic sensilla on the maxillae and one pair of epipharyngeal sensilla on the innerside of the labrum. The reactions of these sensilla were measured by recording the impulses elicited by chemical stimuli using electro- physiological techniques. The number of impulses in the first second after onset of stimulation (Nsp) was counted and served as a measure of sensory activity. Behav- ioural reactions were measured in feeding tests, in which the larvae (after a star- vation period) were fed a test diet containing the stimulus during 24 hours. The dry weight of the faecal pellets produced in this period (Fp) was used to quantify the behavioural reaction. The sensory and behavioural responses were combined in relative input-output diagrams (chapter 2). The responses to the following compounds were determined: sucrose (in both species) inositol, a sugaralcohol (in Mamestra only) sinigrin, representing the glucosinolates, secondary plant substances from the Cruciferae (both species) strychnine, representing the chemically diverse group of deterrents, secondary plant substances that inhibit food intake (both species) and the amino acid proline (Pieris only)