The results of previous studies indicated that block grafts of fetal hippocampal tissue made into cavities produced by aspiration lesions of the hippocampus in rats given the neurotoxin trimethyltin (TMT) significantly worsened the TMT-induced deficit in water maze acquisition. The purpose of the present study was to test the hypothesis that a procedure for transplantation that produced less destruction to the host brain and resulted in transplants with less mass might produce recovery in a spatial learning task in TMT-exposed rats. Acquisition of an externally cued (spatial) version of the radial arm maze (RAM), an internally cued version of the RAM, and of a differential reinforcement of low rate (DRL) operant schedule was assessed in normal rats, rats given TMT, and rats given TMT and stereotaxic implants of either fetal Ammon's horn or entorhinal cortex. The rats receiving Ammon's horn transplants made significantly fewer reentries into the baited arms in both maze configurations and fewer reentries into the nonbaited arms in the spatial RAM than rats in the TMT-only and TMT/entorhinal cortex transplant groups. The rats receiving transplants of Ammon's horn made significantly fewer responses and received more reinforcements during training on the DRL-20 schedule than rats receiving just TMT or rats receiving TMT and transplants of fetal entorhinal cortex. These results support the proposal that transplantation procedures that cause less damage to the host brain and result in transplants that do not occupy a large extent of the ventricular space increase the probability of behavioral recovery.