The purpose of this study was to investigate the reliability of tongue and handgrip strength and endurance measurements in healthy adults using the Iowa Oral Performance Instrument. Fifty-one healthy participants (21 males, 30 females; age range = 19-57 years) were tested on four occasions 1 week apart to determine test-retest reliability. The primary outcome measures were isometric tongue and handgrip strength (best of three trials) and sustained isometric endurance. Small increases (changes in group mean) in both anterior (1.7 %) and posterior (2.5 %) tongue strength and handgrip strength (5 %) between weeks 1 and 2 were observed with no change in subsequent weeks, suggesting that there is only a small learning effect for these measurements. The within-subject variation (mean-typical error expressed as a coefficient of variation [CV]) indicated higher than desirable initial variation for anterior (CV 10.8 %) and posterior (CV 11.8 %) tongue strength and handgrip strength (CV 15.2 %) but this was reduced in weeks 2-4. Intraclass correlation coefficients (ICC) indicated acceptable and improved reliability for both anterior (ICC 0.77-0.90) and posterior (ICC 0.79-0.86) tongue strength and handgrip strength (ICC 0.69-0.91) after week 1. Additional exploratory analyses were conducted with a subset of data to determine whether two values within 5 kPa (tongue) or 15 kPa (handgrip) provide superior strength reliability. Neither tongue nor hand endurance measurements were sufficiently reliable. These findings suggest that tongue and handgrip strength values demonstrate acceptable reliability, especially if familiarization is provided. Further investigation is needed to reduce sources of variability in tongue endurance measurements.