This testimony contains the comments of NIOSH regarding the Advanced Notice of Proposed Rulemaking issues by OSHA in regard to occupational exposures to 1,3-butadiene (106990) (BD). NIOSH continues to recommend that BD be regarded as a potential occupational carcinogen, teratogen, and possible reproductive hazard. NIOSH recommends six studies which should be consulted along these lines. NIOSH is unaware of any studies that have explores the ability of BD to penetrate the skin or that have explored the combined effects of inhalation and dermal exposure. Other aspects considered in the testimony included the methods to be used in estimating the significance of risk at the current exposure level of 1,000 parts per million, particularly mathematical models; the identification of specific quantitative risk assessments of BD; and which tumor incidences in which animal species and by what routes of administration and dose levels should be selected. Concern was also expressed about a possible noncarcinogenic effect of BD on the endocrine system.