Long-term (48-hr) incubations of either the fibroblast strain WI-38 or its SV40-transformed counterpart, WI-38-VA13-2RA, in growth medium containing 1 micron prostaglandin E1 (PGE1) resulted in a sustained production and release of cyclic AMP from the cells into the medium. Despite the steady production, intracellular levels of the nucleotide decreased, reaching steady-state values within 4 hr of the initial exposure to PGE1. These values were maintained for the remainder of the 48-hr experimental period. The steady-state levels of intracellular cyclic AMP were higher than those observed in unstimulated cells, and cyclic AMP-dependent protein phosphokinase was in a highly activated state as compared to controls. Under these conditions little change in the growth or morphology of either the normal or transformed cells was observed. In contrast, inhibition of growth, apparent cell death, and unusual morphological changes were observed in both normal and transformed cells when high concentrations of either PGE1 (10 micron) or the phosphodiesterase inhibitor 1-methyl, 3-isobutylxanthine (0.5 mM to 2 mM) were used, which was indicative of toxic effects of the drugs. It was concluded that cyclic AMP-mediated activation of protein phosphokinase does not completely inhibit growth in WI-38 cells or restore normal growth and morphology to the SV40-transformed cells.