|Abstract||The swim performance assay is a behavioral assessment tool used to measure cardiovascular function in fish. Swim performance is regularly used as a metric in toxicological research with chemicals that are known or suspected to affect cardiovascular function in fish. Previously, the laminar flow assay (LFA) has been the standard method of assessing swim performance in fish to measure their cardiac output. The spinning task assay (STA) is a novel, more accessible method of assessing swim performance; however, previous studies have not compared the two methods. Additionally, there is little documentation of swim performance in larval-juvenile fish of small species, a more sensitive study subject for toxicological research. Therefore, the aim of this research is to compare the swim performance of fish in the LFA to those in the STA to determine which method is better for assessing swim performance in larval-juvenile fathead minnows (Pimephales promelas). In this study, the percent of fish that do not fail to swim increased as age increased in the LFA, but in the STA, there was no correlation between percent failure and fish age. It should be noted that cardiovascular performance can only be quantified if the fish fails to swim in the swimming task. Results show that as fish increase in size, swim performance in the LFA improves, making it a more representative, predictable assay. Results from the STA indicate that swim performance in fish does not improve with size and performance in the STA is not correlated with performance in the LFA. UCRIT values from the LFA have more variation than those from the STA. The results of this study show that the LFA is a more suitable modality for assessing swim performance in larval-juvenile fathead minnows.