No effect of fibrinogen degradation products on specific binding of [3H]spiroperidol and [3H]ADTN was observed. Low molecular weight peptides derived from fibrinogen degradation by plasmin (FDP) increase the motor activity of rats [7] augment apomorphine- and amphetamine-induced stereotypy and diminish haloperidol catalepsy [2]. It has been suggested that FDP interact with central dopamine receptor [2]. Taking the above considerations into account it was decided to investigate the effect of FDP on specific binding of [3H] spiroperidol and 2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene ([3H]ADTN).