1. The effects of p-chloroamphetamine (PCA) (5 mg kg-1, i.p.) on extracellular concentrations of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were monitored in the frontal cortex of conscious, freely moving rats by means of intracerebral dialysis. Concomitant behaviour comprising the 5-HT behavioural syndrome (reciprocal forepaw treading, headweaving, wet dog shakes) and locomotion and grooming was also recorded. 2. PCA markedly and transiently increased extracellular 5-HT but extracellular 5-HIAA values decreased to a moderate extent. 3. The time-course of 5-HT changes correlated positively (P less than 0.001) with those of the three components of the 5-HT syndrome but did not correlate significantly with locomotion and correlated negatively (P less than 0.01) with grooming. 4. As previous work using conventional methodology did not reveal clear relationships between the effects of PCA on 5-HT metabolism and behaviour, the present results emphasise the value of in vivo dialysis in the study of the neurochemical mediation of the behavioural effects of drugs.