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dose depen?Snt manner (Lee et al. 1986b). In ?B earlier study (Lee et al. 1985), As was about 1OX more potent than As in effecting transformation?5 The birth defects were most pronounced in golden hamsters exposed to As during the 24-hour period of critical+gmbryogenesis--i.e., day 8 of gestation (Ferm and Hanlon 1985)--when 1.7 mg As /kg body weight induced neural tube defects in about 90% of the fetuses. Hanlon and Ferm (1986a) showed that hamsters exposed to As and heat stress (39 C for 50 minutes) on day 8 of gestation produced a greater+5percentage of malformed offspring (18 to 39%) than did hamsters exposed to As alone (4% to 8%). TERRESTRIAL PLANTS AND INVERTEBRATES In general, arsenic availability to plants is highest in coarse-textured soils having little colloidal material and little ion exchange capacity, and lowest in fine-textured soils high in clay, organic material, iron, calcium, and phosphate (NRCC 1978). To be absorbed by plants, arsenic compounds must be in a mobile form in the soil solution. Except for locations where arsenic content is high, e.g., around smelters, the accumulated arsenic is distributed throughout the plant body in nontoxic amounts (NAS 1977). For most plants, a significant depression in crop yields was evident at soil-As concentrations of 3 to 28 mg/l of water soluble arsenic and 25 to 85 mg/kg of total arsenic (NRCC 1978). Yields of peas (Pisum sativum), a sensitive species, were decreased at 1 mg/l of water soluble arsenic or 25 mg/kg of total soil As; rice (Oryza sativum) yields were decreased 75% at 50 mg/l of disodium methylarsonate in silty loam; and soybeans (G1 cine max) grew poorly when residues exceeded 1 mg As/kg (Table 3; NRCC 1978). Forage plants grown in soils contaminated with up to 80 mg total As/kg from arsenical orchard sprays contained up to 5.8 mg As/kg dry weight; however, these plants were considered nonhazardous to grazing ruminants (Merry et al. 1986). Attention was focused on inorganic arsenical pesticides after accumulations of arsenic in soils eventually became toxic to several agricultural crops, especially on former orchards and cotton fields. Once toxicity is observed, it persists for several years even if no additional arsenic treatment is made (Woolson 1975). Poor crop growth was associated with bioavailability of arsenic in soils. For example, alfalfa (Medicago sativa) and barley (Hordeum vul are) grew poorly in soils containing only 3.4 to 9.5 mg As/kg, provided the soils were acidic, lightly textured, low in phosphorus and aluminum, high in iron and calcium, and contained excess moisture (Woolson 1975). Use of inorganic arsenical herbicides, such as calcium arsenate, to golf course turfs for control of fungal blight sometimes exacerbates the disease. The use of arsenicals on Kentucky bluegrass (Poa pratensis) is discouraged under conditions of high moisture and root stress induced by previous arsenical applications (Smiley et al. 1985). 39