Elevated cortisol and glucose levels have been reported for fish exposed to a variety of PI-103 371935-74-9 physical and chemical stressors, including metals. The cellular stress response is facilitated by the BAY-60-7550 439083-90-6 action of various stress proteins such as the heat shock protein and the metallothionein families. Heat shock proteins aremolecular chaperones that function by regulating cellular homeostasis through ensuring proper folding, transport, and degradation of proteins. The two main hsp families are hsp70, consisting of the constitutively expressed hsp73 and the stress inducible hsp72, and hsp90. In fish, these proteins are induced in cell lines, primary cell cultures, and whole organisms by a variety of stressors including industrial effluents, pesticides, pathogens, and metals. Exposure to stress causes proteins to denature,misfold, or unfold, ultimately exposing hydrophobic regions and causing protein aggregation. The heat shock response is therefore essential to maintain proper protein structure and cellular function.Metallothionein is a metal binding protein with a high affinity for groups Ib and IIb transition metals. One of the suggested functions of MT is to regulate essential trace metals like zinc and copper and detoxify metals such as cadmium and copper. Synthesis ofMT is induced to a greatest degree by exposure to metals and to a lesser degree by hormones, cytokines, and organic contaminants. The objectives of the present study were to determine the effect of sublethal concentrations of waterborne Mo on the physiological stress response, as measured by plasma cortisol, blood glucose, and hematocrit, and the cellular stress response, as measured by total hsp70, hsp72, hsp90, and MT induction in rainbow trout, a species that has proven to be a very useful model system for understanding the effects of many other metals. Fish in nature are exposed to a variety of stressors that can adversely affect their health. In order to cope with stress, fish can respond to it by eliciting a physiological or a cellular stress response. Such responses result in biochemical, hematological, and cellular changes that can be used as biomarkers to allow for the assessment and management of stress in fish. While there have been many studies correlating various stress related biomarkers with exposure to metals such as cadmium, copper, and zinc, information concerningMo is extremely limited. This study investigated the effects of an acute, sublethal Mo exposure on the stress response of rainbow trout. Both fingerling and juvenile fish did not elicit a physiological or a cellular stress response when exposed to Mo nor were there any detectable differences in sensitivity between the two life stages despite the tissue accumulation of significant molybdenum. Control glucose and hematocrit levels were within the range previously reported in unstressed rainbow trout. The lower hematocrit values observed in the cannulated fish versus the non-cannulated fish are characteristic of cannulated fish. Explanations may be blood loss due to the cannulation procedure and mild hemodilution, caused by repeated sampling and injection of saline after each sampling. In the present study, the absence of hyperglycemia is consistent with the lack of elevated cortisol. Although hyperglycemia was not observed throughout the 96 h exposure period, a hypoglycemia was observed.