Studies were conducted to explore the effects of oleate addition on the secretion of apolipoprotein B (apoB)-containing lipoproteins from Hep G2 cells. Whether oleate was added simultaneously with [3H]-leucine or added to prelabeled cells, the rate of secretion of apoB was stimulated more than 100% within 40 min. When oleate was withdrawn from the cells, the rate of secretion returned to the prestimulated rate within 40 min. These observations suggested that oleate affects apoB secretion early in the secretory pathway. When the effects of oleate on apoB secretion were studied in pulse-chase experiments, it was observed that although apoB synthesis was not affected, apoB intracellular degradation was significant inhibited by oleate. In the absence of oleate, 58% of apoB synthesized during the labeling period was degraded within 20 min, before secretion of apoB into the media had begun, whereas only 29% of labeled apoB was degraded intracellularly during this same time period when oleate was present. Thus, it appears that oleate rapidly stimulates the secretion of apoB by protecting nascent apoB from degradation early in the secretory pathway. Furthermore, stimulation of apoB secretion was observed over a range that includes physiological concentrations of oleate, from 0.1 mM (oleate: bovine serum albumin ratio = 0.45) to 0.8 mM (oleate: ratio = 3.6), suggesting that exogenous oleate could be a physiological modulator of apoB secretion.