Embryonic striatal grafts develop a modular organization in which patches of tissue enriched in many transmitter substances characteristic of striatum (P regions) are embedded in surrounds (NP regions) expressing only low levels of these substances. Catecholaminergic fibers from the host brain, identified by their expression of tyrosine hydroxylase (TH), grow into such grafts and selectively terminate in the striatum-like P regions. This terminal pattern suggests that cell-cell affinities between neurons of the substantia nigra and striatum may play a role either in the aggregation of the striatal cells into P regions, or in the targeting of the TH- positive fibers to the cell clusters. In the present study, we tested the first of these possibilities. Striatal grafts derived from embryonic day 15 striatal primordia were implanted into the ibotenate- damaged host striatum of rats previously treated with 6-hydroxydopamine (6-OHDA) to destroy TH-containing dopaminergic nigrostriatal afferents. The 6-OHDA lesions that eliminated nearly all TH-like immunostaining in the host striatum also resulted in disappearance of nearly all TH- positive fibers in the grafts. In this dopamine-depleted environment, the grafts nevertheless developed a clear modular organization. They contained striatum-like patches with neurons expressing many of the neurochemicals characteristic of striatum (ACh, ChAT, calbindin-D28KD, met-enkephalin, and dopamine- and adenosine 3′:5′-monophosphate- regulated phosphoprotein-32,000 or DARPP-32), and these patches were surrounded by graft tissue expressing few of these striatal markers. These observations suggest that the ingrowth of TH-positive fibers from the host is not obligatory for the sorting out of striatal from nonstriatal cells during the formation of P regions in embryonic striatal grafts. Despite the fact that dopaminergic denervation of the host striatum did not disrupt either the aggregation of grafted cells into P regions or the acquisition of striatal neurochemical phenotypes by cells in the P regions, there were clear differences between the staining patterns of these grafts and grafts placed into dopamine- innervated striatum. Most striking was a sharp increase of met- enkephalin-like immunostaining in the P zones of the denervated grafts. Upregulation of met-enkephalin is known to occur in the dopamine- depleted mature striatum, and was observed in the parts of host striatum surrounding the grafts on the side ipsilateral to the 6-OHDA lesions. This result suggests that functional interactions between dopaminergic and enkephalinergic systems can occur in the striatal circuits reconstructed by embryonic striatal grafting. More generally, our results suggest that TH-containing afferents from the host striatum, though not required for induction and maintenance of striatal phenotypy in striatal grafts, can chronically regulate neurotransmitter/neuromodulator expression in neurons of the striatum- like P zones in a manner similar to that found for the normal striatum.