The immunocytochemical localization of tyrosine hydroxylase is examined during early ontogeny in the fetal rat brain in order to determine the age of first detection and subsequent cellular localization of the enzyme and the developmental characteristics of the immature catecholaminergic neurons. Fetal atlases of the tyrosine hydroxylase-labeled neurons are presented at embryonic day (E) 12.5, 13.5, and 14.5. Tyrosine hydroxylase is first detected immunocytochemically at E 12.5. At this stage, the labeled neurons have completed final mitosis, but are still migrating and are cytologically immature. Tyrosine hydroxylase can also be detected in axons and axonal growth cones at this stage of development. The age of first immunocytochemical detection of the enzyme precedes the demonstration of catecholamine fluorescence by 1 to 2 days in certain nuclear groups. At later stages of development (E 13.5 and E 14.5), the major groups of perikarya and processes labeled for tyrosine hydroxylase have a distribution similar to that previously described by catecholamine fluorescence. At E 14.5, the perikarya undergo considerable changes in their cytology and exhibit the first dendrites immunocytochemically labeled for the enzyme. The first terminal fields are also detected in the rudimentary caudate-putamen at this stage.