How is axonal transport in regenerating neurons affected by contact with their synaptic target? We investigated whether removing the target (homotopic) lobe of the goldfish optic tectum altered the incorporation of 3H-proline into fast axonally transported proteins in the regenerating optic nerve. Regeneration was induced either by an optic tract lesion (to reveal the changes in the original axon segment that remained connected to the cell body) or by an optic nerve lesion (to reveal the changes in the newly formed axon segment). Of 26 proteins analyzed by 2-dimensional gel electrophoresis and fluorography, all but one showed increased labeling as a result of tectal lobe ablation. By 2 d after the lesion, significantly increased labeling of some proteins was seen with a 6-hr labeling interval, but not with a 24-hr labeling interval. This is probably indicative of an increased velocity of transport, which may have been a nonspecific consequence of the surgery. Otherwise, tectal lobe removal had relatively little effect until 3 weeks, when there was a transitory increase in labeling of transported proteins in the new axon segments of the tectum-ablated animals. Beginning at 5 weeks, tectal lobe ablation caused considerably higher labeling of many of the proteins in the original axon segments. Because this was seen with both 6-hr and 24-hr labeling intervals, it is probably indicative of increased protein synthesis. The increased synthesis lasted until at least 12 weeks, though some proteins were beginning to show a diminished effect at this time. In the late stages of regeneration (8-12 weeks), there was also increased labeling of proteins in the new axon segments as a result of the absence of the target tectal lobe. This included a disproportionately large increase in the relative contribution of cytoskeletal proteins and of protein 4, which is the goldfish equivalent of the growth-associated protein GAP- 43 (neuromodulin). We conclude that, after the regenerating axons begin to innervate the tectum, the expression of most of the proteins in fast axonal transport is down-regulated by interaction between the axons and their target. However, the changes in expression may be preceded by a modulation of the turnover and/or deposition of proteins in the newly formed axon segment.