With the purpose of detecting spontaneous variation of systolic time intervals (STI), 20 normal subjects have been examined and the STI has been recorded for 5 consecutive days at the beginning of the test and 20, 40 and 60 min afterwards. Significant differences were found for PEP, LVET and the PEP/LVET ratio between observations (p less than 0.05) and between days (p less than 0.01). All the values of the STI at the fifth day are rather near the values recorded at the last observation of the first day. This may be related to the presence of some factor (catecholamine release induced by emotional stress?) resulting in physiological changes which are reduced by repetition of the test. This hypothesis was confirmed in a second series of normal volunteers where the STI were recorded simultaneously with sampling of blood for assessing circulating CA levels. A highly significant correlation (p less than 0.001) was recorded between PEP, PEP/LVET and plasma CA making it evident that increase of PEPI and PEP/LVET is directly correlated with the reduction of plasma CA level.