The purpose of this work was to design an in vitro model for studying the lipolysis of very low density lipoprotein (VLDL) under conditions that approximate those likely to exist in the rat circulation. We first measured the total lipase activity available in the circulation of normal fasted rats. Knowing the VLDL-triacylglycerol concentration and the circulating time of VLDL in rat serum, we calculated that incubating 80 mU of lipoprotein lipase activity per milligram of VLDL triacylglycerols at 37 degrees C, for 50 min, approximated the in vivo conditions of lipolysis in normal fasted rats. Under these conditions, different concentrations of albumin were tested. The degree of lipolysis gradually increased from 25 +/- 10% without albumin to 75 +/- 5% (mean +/- SD, n = 3) with 4% albumin. No further increase occurred above 4% albumin. The rate of triacylglycerol hydrolysis was faster with VLDL having a high triacylglycerol/protein ratio. However after 50-60 min of incubation, the extent of hydrolysis tended to become similar for all VLDL (75-90%). This suggests that the rate of VLDL hydrolysis differs from one particle to another (depending on the size), but the extent of hydrolysis ends up being approximately the same. Furthermore, addition of high density lipoprotein to the incubation medium did not affect the rate nor the extent of triacylglycerol hydrolysis. We conclude that a large proportion of VLDL triacylglycerols can be hydrolyzed in the normal fasted rat if there is no limitation in the capacity of fatty acid removal from the lipolysis site. The physiological significance of these results obtained in vitro is discussed.