It has been shown previously that tectal tissue obtained from young embryoscan be successfully transplanted to the neonatal rat brain. In the present study, tecta from E15 rat embryos were maintained as free-floating explants for 3-14 days in vitro (DIV) before being transplanted to the midbrain of new brain rats. We wished to determine how explant culture affected (i) graft survival, (ii) the subsequent morphological and histochemical development of tectal grafts and (iii) the specificity with which host retinal and cortical axons grew into and innervated the graft neuropil. Grafts were examined 6-40 weeks posttransplantation. Host retinal input was assed by injecting the host eyes with either [3H]proline, horseradish peroxidase (HRP) or wheat-germ agglutinin conjugated HRP. The host cortical projection was examined using anterograde degeneration techniques. Frozen tissue sections were also stained for Nissl, neurofibrils or reacted for acetylcholinesterase (AChE). All 3 DIV and 7 DIV explants survived transplantation and many grew considerably in size within the host brain. 14DIV grafts were smaller and were found in only 50% of host brains. The cellular organization, fibre architecture and pattern of AChE staining in cultured grafts was similar to that found in non-cultured tectal transplants. Furthermore, host retina and cortex projected into the grafts in a manner similar to their innervation of non-cultured tectal tissue. The data show that (i) a proportion of most, if not all types of developing tectal neurones survive in vitro in the absence of extrinsic non-tectal afferents and (ii) at least some of these neurones continue to express specific properties which allow them to be selectively recognized by ingrowing host retinal and cortical afferents after transplantation.