The rates at which venomous animals produce venoms are of obvious biological and medical importance, but factors inﬂuencing those rates remain poorly understood. We gathered data on venom yield (wet mass of venom) and percentage solids (dry mass of the venom divided by wet mass) for 53 eastern brownsnakes (Pseudonaja textilis ) and 36 mainland tigersnakes (Notechis scutatus ) over a 4-year period at Venom Supplies Pty. Ltd, a commercial venom production facility in South Australia. Tigersnakes yielded about threefold more venom (by wet mass) than brownsnakes, but with slightly lower percentage solids. Both species showed signiﬁcant geographic variation in percentage solids. Venom yields varied as a function of the snake’s sex and geographic origin, but these effects were secondary consequences of geographic and sex-based differences in body size. Relative head size affected venom yield in brownsnakes but not tigersnakes. Overall, the amount of venom that a snake produced during milking was affected by its species, its geographic origin, its body size and relative head size, and by the time of year that it was milked, as well as by interactions among these factors. Body size was the most important effect on venom yield, with yields increasing more rapidly with size in brownsnakes than in tigersnakes. Research at the intersection of snake ecology and venom characteristics has great potential, but will require a genuinely interdisciplinary approach. q 2002 Elsevier Science Ltd. All rights reserved.