All modalities of toxicity exhibit a dosage-response relationship. Since the quantitative importance of various factors other than dosage and occasionally species and sensitivity that modify the toxicity of a given compound is limited, it follows that real assurance about the possible long-term effect of small repeated doses may be gained by studying the effects of larger doses given over a briefer period. This is particularly true if the potential problem of important species difference can be overcome by studies in volunteers and in workers whose exposure is more intensive and often more continuous than that of the general population. It is especially amoung such workers that a meaningful search may be made not only for the recognized effects of a particular compound but also for effects that are suspected but undemonstrated. Any compound may present a hazard if it is misused, and the modern pesticides are no exception. However, there is strong and growing evidence that long-term absorption of traces of one or more of the synthetic nonmetalic pesticides will not lead to illness.