Experimental measurements of basic transmission loss have been made between 4 and 32 MHz on a 235 km over-ocean path. The purpose was to verify theoretical calculations of ground wave basic transmission loss, including the effect of varying sea state as postulated by Barrick. Stepped frequency soundings were taken two times per day for a one-month period, and hourly for one 24-hour period. In general, the data compare well with theory. The variations in signal strength during the 24-hour test at frequencies below 20 MHz are consistent with sea state change and somewhat correlated with the recorded winds. However, at frequencies above 20 MHz the variations in signal strength were quite different, and (at 32 MHz) gave signals at times 25 dB greater than predicted by smooth earth theory. This phenomenon was attributed to ducting.