Adult zebrafish were placed in a small beaker of aquarium water supplemented with 5 μM 4-HT for 12 hr and subsequently transferred to fresh 4-HT-containing medium for an additional 12 hr. Then, the fish were rinsed with fresh aquarium water and returned to the recirculating water system. Vehicle- or 4-HT-treated fish were used for regeneration experiments or epicardial cell isolation 3 days after the treatment.