In the last part of this series we have studied the effects of various drugs on ponto-geniculo-occipital (PGO) waves induced by the benzoquinolizine derivative, Ro 4-1284 (PGO(1284)), and by the inhibitor of trypotophan hydroxylase, p-chlorophenylalanine (PGO(PCPA)), and continuously recorder and counted in the lateral geniculate bodies (LGB) of unanaesthetized and immobilizedcats. The major aim of this study was to test the specificity of drug-induced alterations of the PGO wave activity suggested by the previous investigations. Hypnotics-sedatives of different classes had no significant effects in doses that did not markedly alter the electrical background activity in the LGB. A notable exception was gamma-hydroxybutyric acid which increased the density of PGO(1284) and PGO(PCPA). A number of neuroleptics were found inactive; sulpiride surprisingly decreased the density of PGO(1284). Bulbocapnine had a similar effect. Convulsants in subconvulsive doses did not uniformly affect PGO waves; while pentetrazole had no consistent effect, strychnine decreased and picrotoxin increased the density of PGO(1284). High doses of morphine, methadone and meperidine decreased the PGO(1284). Ethanol was inactive even in high doses. Caffeine and mefexamide reduced the density of PGO(1284). Mepiprazol was the most potent depressant of PGO(1284, probably by inhibiting the uptake of 5-HT. Mescaline was a weak depressor of PGO(1284). p-Chloromethamphetamine induced PGO waves in untreated cats less consitently than did PCPA. Amantadine reduced the amplitude of PGO waves due to a central antinicotinic action. The results of this study and of the whole series suggested a tentative scheme of the generation and modulation of PGO waves, in which the hypothetical roles and sites of action of four central neurotransmitters are included.