In this study we have used the rainbow trout as a model animal to study the biological consequences of stress in terms of gamete quality and quantity. Groups of 30 mature male and female rainbow trout were subjected to repeated acute stress during the 9 mo prior to spawning. Time of ovulation, fecundity, and egg size were recorded in mature females, and sperm counts were carried out on the milt from the male fish, from both the stressed and control groups. Eggs from ovulated females were fertilized with milt from males subjected to the same treatment regime. Approximately 300 eggs from each female were fertilized with a sperm dilution of 10(-3) in diluent. Subsequent development of the fertilized eggs was then monitored. There were no differences in somatic weight or length between the two groups at the end of the experiment, but exposure of rainbow trout to repeated acute stress during reproductive development resulted in a significant delay in ovulation and reduced egg size in females, significantly lower sperm counts in males, and, perhaps most importantly, significantly lower survival rates for progeny from stressed fish compared to progeny from unstressed control fish. Hence, stress reduces the quality of gametes produced by rainbow trout.