The age related changes on free plasma testosterone (T), dihydrotestosterone (DHT) and oestradiol (OE2) were determined in 82 healthy adult males. Forty-six subjects were between 22 and 61 years of age (group I), 36 between 67 and 93 years (group II). The percentage of free, non protein bound hormone was determined by equilibrium dialysis of undiluted plasma against isotonic phosphate buffer at 37 degrees C. Total hormone concentrations were measured by radioimmunoassay. The percentage of free T was 2.24% (median), 1.65-3.42 (95 percentiles) in group I and 1.65% (1.24-2.26) in group II. The percentage of free DHT decreased from 1.17% (0.80-2.03) in group I to 0.83% (0.52-1.55) in group II. The decrease in % free Oe2 was only very small: group I = 2.49% (2.13-2.96), ad group II = 2.31% (1.95-3.17). The fall in free T by 43.3% from 12.2 ng/100 ml (6.74-25.0) in group I to 6.90 ng/ml (3.57-10.6) in group II was twice as high as that of total T, which decreased on an average by 20.6%. Free DHT decreased by 25.8%: group I = 578 pg/100 ml (266-987), group II =429 pg/100 ml (168-723), while total DHT was not significantly different between the two groups (-1.9%). Free Oe2 was increased in old age: group I = 42.4 PG/100 ML (26.0-69.4), GROUP II = 55.7 PG/100 ML (35.8-118.9). The increase in free Oe2 by 31.4% was almost as high as that of the total Oe2 (46.9%).