[H-3]-serotonin uptake in platelets was studied in 26 patients with se
nile dementia of the Alzheimer type (SDAT), 29 age-matched normal elde
rly persons, and 21 young subjects. The results showed a significantly
lower uptake of serotonin into platelets of SDAT patients than in tho
se of elderly controls (p < .02). Uptake of serotonin into platelets o
f elderly normal subjects was significantly lower than in the young (p
< .01). These differences were due to reduced Vmax, whereas Km was un
changed. Isolated plasma from SDAT patients did not affect the uptake
of serotonin into platelets of elderly controls, and vice versa, i.e.,
plasma obtained from elderly controls did not affect serotonin uptake
in SDAT patients. The results indicate that serotonin uptake into pla
telets is reduced in normal aging, and more so in SDAT. Moreover, the
reduced uptake in SDAT is not caused by a plasma factor in SDAT patien
ts. No correlation was found between serotonin uptake and degree of co
gnitive impairment in SDAT patients.