A computational neural model of movement production in normal and Parkinson's disease (PD) is used to provide a neural account for the source of micrographia in PD handwriting. It is hypothesized that smaller than normal pallido-thalamic signals, due to dopamine depletion, are responsible for the observed overall smallness, slowness and variability in PD handwriting. Experimental data from PD patients that show micrographia support this hypothesis and imply the functional segregation of basal ganglia neural populations.