BALTIMORE (AP) _ Dr. Curt Paul Richter, credited with discovering the biorhythms that regulate sleep and other basic body functions, died of natural causes at age 94.

Richter, a professor emeritus of psychobiology at Johns Hopkins Medical Institutions, died Wednesday at a retirement community here, said Hopkins spokesman Phil Kibak.

Richter demonstrated during more than 50 years of research on animals and humans that some behavior is driven by internal mechanisms operating in predictable cycles, said Dr. Paul McHugh, director of the department of psychiatry and behavioral science at Johns Hopkins University School of Medicine.

Richter determined that such drives as hunger, sleep, thirst and the need for particular nutrients were part of the body's mechanisms and he discerned that those mechanisms were driven by portions of the brain.

His work evolved into the study of biorhythms, or the cyclical nature of the body in controlling basic body functions, such as sleep.

''Before Richter's work, everyone agreed that the body was driven by instincts, but lacked a sense of how to investigate the body in that regard,'' McHugh said.

Richter's 1927 paper on animal behavior and the internal mechanisms that drive their behavior is widely regarded as the primer that spurred modern research on how behavior affects health.

Richter is also credited with discovering that behavior will automatically seek to keep the body nutritionally balanced.

In a series of what became known as the ''cafeteria experiments,'' Richter deprived mice of particular nutrients, such as potassium and salt. He then set before the animals various foods, and found that the mice ate the particular foods that offered high concentrations of nutrients their bodies lacked, McHugh said.

''He demonstrated the wisdom of the body expressed in behavior,'' McHugh said.

The Denver native earned an undergraduate degree in 1917 from Harvard University and a doctorate from Johns Hopkins in 1921.

Richter served as director of the psychobiology laboratory in the Henry Phipps Psychiatric Clinic of Johns Hopkins Hospital from 1922 until 1957, when he was appointed professor of psychobiology at Hopkins medical institutions.

After retiring in 1960, he continued to work in his laboratory until this year.

He wrote more than 250 research papers and received honorary degrees from the University of Chicago, Hopkins and the University of Pennsylvania.

He also was the recipient in 1977 of the prestigious Passano Award, given each year to those who make outstanding contributions to the advancement of medical science.

''He was a great natural scientist who would get simple experiments out of looking at the behavior of animals, and those simple experiments illuminated for us the biological control of the instincts,'' McHugh said.

Richter is survived by his wife, a son and two daughters.

A memorial service was scheduled for Friday.