The PH gradients, oxygen partial-pressure gradients and growth curves were measured for 7 different types of spheroids. Growth curves were measured in liquid overlay culture and thereafter the spheroids were attached to cover glasses and transferred to a chamber for micro-electrode measurements. The spheroids were randomly divided for PH or p02 measurements which then were made under conditions as identical as possible. The decreases in p 0 2 and pn. Ap02 and APH were calculated as the difference between the values in the culture medium and the values 200 pm inside the spheroids. Each type of spheroid had a certain relation between Ap02 and Apn. The human colon carcinoma HT29, the mouse mammary carcinoma EMT6 and the hamster lung V79-379A spheroids had high values of the quotient Ap02/Ap~. The human thyroid carcinoma HTh7 spheroids and the 3 types of human glioma spheroids had lower quotients. There was a tendency for fast-growing spheroids to have high quotients. Two extreme types of spheroids, HT29 (high quotient) and U-I18 MG (low quotient) were analyzed for lactate production and oxygen consumption. The U-I18 MG spheroids produced about 3 times more lactate and consumed about 3 times less oxygen than the HT29 spheroids. The differences in lactate production could not be explained by differences in the pyruvate Km values of lactate dehydrogenase. The results indicate that there are significant metabolic differences between the spheroid systems studied. 3T0 whom reprint requests should be sent.