A commercially available human skin equivalent (HSE) was used as an in vitro organotypic skin model to study temporal morphological effects of sulfur mustard gas (HD). Light and electron microscopic analyses of the HD-human skin equivalent model (HD-HSE) were compared to the HD-hairless guinea pig model (HD-HGP). HSE samples were exposed to 10 microliters HD vapor for 8 min and harvested at selected times up to 24 h. Skin sites of HGP were exposed to the same vapor dose or to 2.0 microliters liquid HD for 30 min and collected at 12 and 24 h. In both models, basal cells of the stratum germinativum were selectively affected. The HD-HSE study revealed that basal cell changes began 3 to 6 h following exposure. These early cellular changes included an acantholysis of some basal cells with widening of intercellular spaces, disruption of desmosomal attachments, nuclear pyknosis, perinuclear blebbing and repositioning of cytoplasmic tonofilaments to a perinuclear position. At 12 to 24 h, basal cell pathology progressed to diffuse swelling of endoplasmic reticula, cytoplasmic vacuolations and necrosis which now extended to supra basal cell layers. Comparing basement membrane zone effects, HD-HGP consistently developed characteristic microblisters at the dermal-epidermal junction; however, HD-HSE with its absence of a morphologically distinguishable basement membrane did not. Instead, cellular fragments, granules and debris accumulated early in this area to thicken regions usually assigned to the lamina lucida and lamina densa of a true basement membrane leading to complete separation of dermis from epidermis at later time periods.