J. Murdoch Ritchie, a biophysicist at Yale who used a potent neurotoxin derived from shellfish to help trace the way nerve cells conduct electrical impulses and famously asked the Central Intelligence Agency to share its supply of the poison with scientists, died on July 9 in Hamden, Conn. He was 83.

Dr. Ritchie’s death was confirmed by his family.

In 1975, while a Senate Select Committee on Intelligence was reviewing the C.I.A.’s “operational use” of poisons, Dr. Ritchie asked for access to the agency’s store of saxitoxin, a rare and highly effective neurotoxin made by clams.

The request raised eyebrows in Congress because the substance, which kills by causing respiratory failure, was not even supposed to exist; President Richard M. Nixon had ordered the government to destroy all of its bacteriological weapons in 1969. To the committee’s dismay, the C.I.A. did not turn over its saxitoxin supply, which the agency said was used to prepare suicide pills for spies in case of capture.

In the early 1970s, Dr. Ritchie, who was known as Murdoch, used saxitoxin for a nonlethal purpose, in studies of electrical conduction within nerve cells. It was already known that the nervous system used shifts in levels of sodium and potassium to transfer electrical signals and that saxitoxin could be employed to block the movement of sodium.