The high density lipoprotein HDL) response of 14 hyperlipidemic subjects to four hypolipidemic agents was studied through serial measurement of HDL cholesterol and apolipoproteins A-I and A-II before and during 3 months each (separated by 2 months off drug) of clofibrate (2 g/day, n = 14), colestipol (20 g/day, n = 12), para-amino salicylic acid--ascorbate (PAS-C, 6--8 g/day, n = 14) taken in random sequence and oxandrolone (7.5 mg/day, n = 11) as the final drug. The maximal effect of each drug appeared by the first monthly evaluation, and A-1, A-II and HDL cholesterol levels returned to pretreatment levels by one month after discontinuation of each agent. With clofibrate, HDL cholesterol increased by 16 +/- 20% from baseline (mean +/- SD) (P less than 0.05), A-I by 11 +/- 13% (P less than 0.05) and A-II by 39 +/- 17% (P less than 0.01). During oxandrolone HDL cholesterol declined by 36 +/- 20% from baseline (P less than 0.01), A-I by 21 +/- 13% (P less than 0.01), and A-II by 16 +/- 11% (P less than 0.025). Neither PAS-C nor colestipol exerted major effects on HDL, or any of the variables although both were associated with a slight rise in the A-I/A-II ratio (11 +/- 15% and 12 +/- 12%, respectively).