Serotonin (5-HT) is an important neurotransmitter involved in the brain-gut axis. It is possible to lower the 5-HT level in the body by means of a nutritional intervention using an amino acid mixture; the acute tryptophan depletion (ATD) method. We studied the effect of ATD on gastric emptying in healthy females, who received both ATD and placebo in a random order. Gastric emptying was measured using the [(13)C]octanoic acid breath test. The present data demonstrate significant differences in both gastric emptying and lag phase (Tlag) between the ATD and placebo experiment. Eight out of ten subjects showed a delayed gastric emptying in the ATD experiment. Both the gastric half-emptying time (T1/2) and the Tlag were significantly higher in the ATD experiment. T1/2 in the ATD experiment was 137.2 (range 76.2-634.8) min; T1/2 for the placebo experiment was 98.5 (range 63.7-168.8) min (P=0.028). Tlag in the ATD experiment was 83.7 (range 45.1-356.2) min; Tlag for the placebo experiment was 56.9 (range 23.2-101.2) min (P=0.007). We conclude that lowering the 5-HT level in the body using the ATD method leads to a significantly delayed gastric emptying of a solid meal. Nutritional manipulation of the serotonergic system in healthy volunteers may lead to alterations in gastrointestinal motility.