The human small cell lung cancer cell line NCI-H82 was used to study the effect of nutritional status on cell proliferative parameters. Incorporation of bromodeoxyuridine (BrdUrd) was used to characterize actively proliferating cells and to obtain information on cell cycle dynamics. During several days, in which the culture medium was not changed, a gradual decrease in overall cell growth, labeling index, and vitality was observed. Simultaneously, an increase in the number of S-phase cells that did not incorporate BrdUrd was noticed. From a more detailed kinetic study on d 6 of nutrient depletion, it appeared that, although the cells incorporated BrdUrd, they stopped cycling. When the same cells were regrown in fresh culture medium, a delay of 10 h in G1-phase entry and exit was measured. After this delay the cells resumed the cell cycle at normal phase transit rates. In addition, BrdUrd unlabeled S-phase cells were gradually lost from the culture. Bivariate flow cytometric DNA/proliferating cell nuclear antigen (PCNA) and DNA/Ki67-antigen analyses confirmed a delay in G1 phase entry and exit. In this paper we show that nutrient depletion can cause cell cycle arrest as indicated by the occurrence of BrdUrd unlabeled S-phase cells. This arrest could lead to overestimation of kinetic parameters such as S-phase transit time (Ts) and potential time (Tpot) as determined after in vivo labeling of tumors.