Ventral mesencephalic anlagen survive following grafting to the striatum of weaver mutant mice and reinnervate the dopamine-depleted basal ganglia of the recipients. The aim of the present study was to examine the pattern of connectivity established by graft-deriving dopamine afferents in the host striatum. Grafts were obtained from normal embryos at a gestational age of 14-15 days and implanted into a surgical cavity overlying the dorsal striatum of adult weaver recipients. Tissue was processed for electron microscopic immunocytochemistry using a primary antiserum against tyrosine hydroxylase. At the time of examination, recipient weaver mutants were 8.5 months old and the grafts had survived for 4.5 months. Grafts were found to contain an estimated 100-1000 tyrosine hydroxylase immunoreactive neurons. Tyrosine hydroxylase immunoreactive fibres, displaying characteristic varicosities, innervated the dorsal striatum to a depth of 1000 μm. In the non-grafted striatum, 8% of the contacts of tyrosine hydroxylase immunoreactive nerve terminals were junctional. That proportion contrasted with the corresponding value of normal animals, which is 27%. In the grafted striatum, 29% of the contacts were junctional. That percentage approximated the value found in normal animals. By applying a stereological correction, it can be estimated from those numbers that the true proportion of junctional contacts in the non-grafted striatum of 8.5-month-old mutants may be 26%, whereas that in the grafted side may be 91%, which is close to the normal situation. The majority of contacts in the reinnervated striatum (84%) were made with dendrites and spines. However, the proportion of total axosomatic contacts in the reinnervated striatum was twice as high as that found in the striatum of normal animals, and the proportion of junctional synapses was three times higher than that found normally. We conclude that: (1) in spite of a genetically determined degenerative process, the dorsal neostriatum of weaver mutant mice is receptive to synaptic investment by dopamine afferents originating in normal donor tissue. (2) In repopulating the denervated weaver striatum, graft-deriving dopamine afferents display a connectional selectivity, i.e. they establish synaptic relations preferentially with those cellular domains that are normally innervated by dopamine nerve terminals. In this context, it is possible that dopamine fibres originating in the grafts invest postsynaptic sites that had either been vacated from the intrinsic dopamine input or had never received such an input. (3) The striatal connectivity following transplantation may retain features of immaturity as suggested by the increased incidence of axosomatic contacts, a feature of the developing nigrostriatal projection.