The selective labeling of serotonin (S) receptors in caudate by 3H-LSD was studied. Mehods Caudate homogenates were prepared from calf brainE and - used to study S-specific LSD(200 nM) binding. An APS (mixture containing 50 nM apomorphine, phentolamine and spiperone) was also used to prevent binding to a-adrenoreceptors or dopamine receptors. Various drugs were used as competitors for LSD. Results By Scatchard analysis the number of LSD bindine sites was decreased from 1100 fmol (without APS) to 300 fmol,7mg protein (with APS); Kd values were 8.2 nM without APS and 4.8 nM with APS. The IC50 for S against 3H-LSD with APS and 4.8 nM with APS. The IC50 for S against 3H-LSD with APS was 35 nM (1000 nM without APS), thus 3H-LSD was more selectively displaceable by S and the effects of norepinephrine and dopamine were eliminated. The effect of several competitive drugs on 3H-LSD binding in the preacnce of APS was studied. Active drugs were dopamine, serotonin, bufatenine, 5 -methoxydimethyltryptamine, psilocin, tryptamine, N,N -dimethyltryptamine, 5,6-dihydroxytryptamine, dihydroergotamine, LSD, methysergide (Sandoz), mete rgoline (Farmitalia) brom-LSD, dibydroe rgoc ryptine, methiothepin, mianserin (Organon), quipazine (Miles), (+)butaclamol (Ayers") chlorpromazine, imipramine (CIBA-Geigy), desipramine, nortriptyline, haloperidol, propranolol, amitriptyline, bromocriptine and drugs having no effect at 3500 nM were atropine, carbachol, deanol, phenytoin, fenfluramine (Robins). fluoxletine, harmaline, ibogaine, meprobamate, mescaline, morphine, nialamide, nicotine, acetylcholine, adrenaline, GABA, histamine, noradrenaline, tryptophan, 5-HIA, indole, melatonin and (-)-butaclamol (Ayers").