A complete diallel cross using both sexes was derived from four parental populations of chickens having divergent developmental rates. This paradigm was used to investigate the genetic architecture of the neurochemicals, norepinephrine (NE), epinephrine (EPI), 3,4-dihydroxyphenylacetic acid (DOPAC), dopamine (DA), and 3-methoxy-4-hydroxyphenylglycol (MHPG). No sex differences were found for any genetic parameter investigated. Estimates of average line effects, maternal effects, and additive sex linkage were not significant. Highly significant heterosis, in the form of overdominance, was observed for catecholamine and metabolite concentrations. Hybrids exhibited significant line heterosis in the positive direction for NE, while heterosis for MHPG was observed in the negative direction for all crosses. Heterosis for EPI was both line and brain area specific with the hypothalamus showing greatest heterotic effects. DOPAC showed significant heterosis for all lines only in the optic tectum, and DA showed little heterosis specific to any line, cross, or brain area.