The relationship between plasma cyclic nucleotides and the "Yin gang Theory" was investigated. 247 patients with various diseases were selected by doctors experienced in traditional Chinese medicine when their clinical picture showed typical "Yindeficiency" or "Yang-deficiency" syndrome Plasma cAMP and cGMP contents were determined before clinical treatment using competitive protein binding assay （for both cAMP and cGMP） or radioimmunoassay （for cGMP in the late stage of this work）. The main resuhs may be summarized as following:1. As shown in the table

in "Yindeficiency" patients

the plasma cAMP level increased significantly

and this was true for all the diseases studied

cGMP contents were either normal or only slightly elevated so that the cAMP/cGMP ratio was within normal range （in most diseases） or slightly increased （in endocrine diseases

mainly hyperthyroidism）. 2. In "Yang-deficiency" patients

the changes may be divided into two types. In the first type （group Ⅰ）

the plasma cAMP decreased significantly

while the plasma cGMP increased slightly. In the second type （group Ⅱ）

both the cAMP and cGMP contents increased significantly

but the increase of the latter was far more remarkable. As a result

the cAMP/cGMP ratio of "Yang-deficiency" patients was much lower than normal in all the diseases studied.3. The plasma cyclic nucleotide contemts of 35 patients of group Ⅰ of the "Yangdeficiency" type were followed after a period of treatment. Chi-square test revealed a definite relation between the clinical progress and the cAMP content and cAMP/ cGMP ratio （i. e.

cAMP content and cAMP/ cGMP ratio increased when the clinical syndrome improved and vice versa）. 10 patients of the "Yin-deficiency" type ware also followed after treatment and a reverse relation between the cAMP content and the clinical progress was obtained （cAMP level decreased along with the clinical improvement and vice versa）.4. According to the "Yin-Yang Theory"

"Yin-deficiency" and "Yang-deficiency" are due to impairment of the balance of "Yin" （阴） and "Yang" （阳）. It is therefore suggested by the authors

from the data of this work

that the impairment of the balance between cAMP and cGMP may be a fundamental change in "Yin-deficiency" and "Yang-deficiency" —— In "Yindeficiency"

cAMP is in predominance

and in "Yang-deficiency"

cGMP is in predominance. The relation between the various physiological actions of the two cyclic nucleotides and the various symptoms and signs of the two "deficiency" syndromes and the possible role played by cAMP and cGMP in causing these syndromes are discussed.