Five control chick embryos and fifteen exposed to LSD at concentrations of 2.5, 12.5 and 50 μg/ml were examined. The morphometric analysis performed at the light microscope level shows that ( 1) the section area of the neural tube increases with the two highest doses of lysergic acid diethylamide (LSD) employed, ( 2) the volume fraction of the intercellular spaces decreases with the two highest doses of LSD, ( 3) the volume fraction of the nuclei in the neural tube is not modified with any concentrations of LSD employed, ( 4) the volume fraction of the cytoplasm in the neural tube increases with all three concentrations, and ( 5) the nucleo-cytoplasmic ratio decreases with all three doses employed. Moreover, at the ultrastructural level, it was found that ( 1) the volume fraction of mitochondria in the cytoplasm decreases at doses of 12.5 and 50 μg/ml ( 2) the surface to volume ratio of the mitochodria is unchanged with any of the concentrations of LSD employed and ( 3) the surface density of the endoplasmic reticulum in the cytoplasm increases only with a dose of 2.5 μg/ml of LSD.