A specialist in internal medicine testified here today that James Bock, a patient for five yars at a private psychiatric hospital in Rockville, was suffering from a physical illness that the hospital never diagnosed or treated.

Bock has sued the hospital, Chestnut Lodge, for $12 million claiming that his psychiatric symptoms - including schizophrenia, paronia, depression, suicidal tendencies, muddled thinking and general lethargy - were caused by hypoglycemia, low blood sugar.

Dr. Ocar Kruesi, a New Jersey internist who teaches medicine at the Rutgers College of Medicine, testified during the fifth day of the trial in federal court here that the records of the lodge and Bock's own medical history dating back to his childhood all indicate he suffered from the condition, which is the medical mirror image of diabetes.

Kruesi testified that Bock's medical records indicated that Bock was given a physical examination when he first entered Chestnut Lodge in 1967 and of sugar. Such a finding, Kruesi testified, places a burden upon the physicians in charge of a patient's care to discover the cause of the sugar findings.

"If you have a trace of sugar, any sugar" you must "give a glucose tolerance test to find out where the sugar came from," testified Kruesi, who provided his services for free, a highly unsual thing for an expert witness to do in a medical malpractice case.

Kruesi testified that the standard of medical care in effect in 1967 dictated that such a follow-up test be given. He then stated that there is no indication anywhere in the records of the lodge that there was such a follow-up.

The internist also testified that Bock's suffering from ulcers at ages 6 and 17, his junk food diet, his severe headaches as a child, his childhood allergies, general lethargy and psychological complaints were all symptomatic of the patient suffering from a severe case of hypoglcemia.

It is Bock's contention that his bypoglycemia was discovered immdeiately after he left the lodge in 1972 and he was cured of his mental illness within 10 days of being placed on a special high-protein, low sugar diet to control the hypoglycemia.

Evidence was introduced today, however, that throws into question Bock's contention that he has been completely cured of his psychiatric problems. During the cross examination of Kruesi, Francis L. Casey Jr., one of the seven attorneys representing the lodge and the five psychiatrists being sued, produced a personality test given to Bock's choosing.

Kruesi testified that the test, the Minnesot Multi-Phasic Inventory, is one he sometimes gives to patients in his general practice to help him decide if they need professional psychiatric help. The tests are graded and evaluated by specialists who know nothing of the patient or the patient's history.

The test results are "strongly suggestive of a major emotional disorder. The test pattern resembles those of psychiatric out-patients who later require in-patient care." the evaluater reparted. "Further evaluation is recommended," the report concluded.

The summary of the report closely resembles testimony about Bock's condition during his stay at the lodge the condition he claims was cured almost five years with a special diet.

To win, Bock must convince the jury that his illness was at least partially caused by hypoglycemia and the Chestnut Lodge had an obligation to test for the condition.

If he wins, the case will set an important precedent because the psychiatric profession has consistently disputed the existence of a link between diet and mental illness.