Summary: The utility of L-tyrosine (10 g/day in four divided doses) as an adjuvant to molindone (150 mg/day) in the treatment of schizophrenia was investigated using a placebo-controlled, double-blind crossover design (3 weeks on L-tyrosine, 3 weeks on placebo). The objective of this inpatient study was to increase dopaminergic neural transmission along mesocortical projections in patients by increasing the precursor availability of L-tyrosine for dopamine biosynthesis. Theoretically, this approach might lessen both negative and positive symptoms of schizophrenia and improve frontal lobe-mediated neuropsychological performance. There was no evidence of statistically significant improvement conferred by L-tyrosine as measured by weekly Brief Psychiatric Rating Scale (BPRS), Schedule for the Assessment of Negative Symptoms (SANS), or Clinical Global Impressions (CGI) scales. The 12-h trough plasma level of L-tyrosine was significantly higher in all patients during the L-tyrosine phase of the study (t = −3.9, df = 20, p = 0.0009). At the end of each 3-week study period, no significant differences could be found in Wisconsin Card Sorting Test (WCST) or memory test performance. Smooth-pursuit eye movement (SPEM) performance had significantly more saccadic intrusions during the L-tyrosine supplementation phase compared to the placebo period. This increase in saccades during SPEM suggests that the tyrosine supplementation might have had some central effect.