We previously reported that qualitative changes in dietary fat influence certain monoaminergic mediated behaviours such as pain sensitivity and thermoregulation in a cold environment after an amphetamine challenge. The purpose of this study was to further explore the behavioural consequences of alterations in dietary fat intake by examining another behaviour known to be mediated by the monoamines--food intake regulation--and to begin investigating a biochemical link between dietary fat composition and behaviour. Rats were stabilized to 20% (w/w) soybean oil (SBO) or lard diets for 10 days and then allowed to select for protein (PRO) and carbohydrate (CHO) intake. While total food intake was unchanged, rats fed the SBO diet selected lower PRO (3.1 +/- 0.6 vs. 4.9 +/- 0.6 g/day, SBO vs. lard, respectively) and higher CHO (9.6 +/- 0.7 vs. 7.8 +/- 1.2) intakes than those consuming the lard based diet. Comparable differences were seen in a second trial. Current evidence suggests that the regulation of PRO and CHO intake is under serotonergic control. Therefore to determine whether dietary fat is mediating its effect on macronutrient selection via alterations in serotonin (5HT) metabolism, brain stem concentrations of 5HT and its metabolite 5-hydroxyindole acetic acid (5HIAA) and whole brain (minus brain stem) mitochondrial monoamine oxidase (MAO) activity were measured in a separate set of animals fed the SBO or lard diets for 28 days. Vmax of MAO was decreased in rats fed the SBO diets (20.2 +/- 7.4 vs. 27.9 +/- 8.9 nmol/mg prot/20') compared to those fed the lard diets. Km was unaltered by dietary fat fed. The change in activity of MAO was insufficient to alter steady-state levels of 5HT or 5HIAA. We propose that changes in neuronal functioning, induced by altered dietary fat, contributed to the differences seen in PRO and CHO selection.