Synaptic ribbons (SRs) are electron-dense, plate-shaped synaptic organelles, to which electron-lucent synaptic vesicles (SVs) are attached by tiny stalks. In the mammalian pineal gland SRs are dynamic organelles, waxing and warning in number under different physiological and experimental conditions. The way in which SRs are formed, or catabolized, is not known. Since it has been suggested that actin may be part of SRs, in the present study the effect of the actin-disrupting drug cytochalasin D (CD, 1 microgram/ml, for 4 h) was examined in cultured guinea pig and rat pineal glands. The glands were preincubated for 48 h so that intra-pineal sympathetic nerve fibres degenerate and no longer release noradrenaline which may distort the results. CD had no effect on SR profile numbers in guinea pigs, but decreased them in rats (p > 0.05). The nonsignificant depressive effect of CD in rats was verified in a second experiment. To clarify the issue, acutely cultured rat pineal glands were treated with CD for 4 h, without effect. The results taken together suggest to us that CD has no major effect on pineal SR profile numbers, but that in rats preincubation for 48 h makes them vulnerable to catabolic processes. In all the experiments, the electron-dense plate of the SRs was qualitatively unaffected. However, the SVs were often larger and more irregular in shape and the stalks linking the SVs with the SRs were less frequently seen in CD-treated glands. In guinea pig pineals, in which SRs frequently lie in groups and parallel to each other, neither the distance between neighboring SRs nor the thickness of individual SRs were affected by CD. It is concluded that actin is not a major component of the SRs and the connecting stalks.