Many radioactive elements have existed when the Earth had appeared and the long living of them are still present. One of the most important of these radioactive elements is 40K. It makes about 0.012% of total content of K, widely represented in many different rocks. In this study the distribution of the natural 40K radioelement in the volcanics of the south part of Afyon was investigated. Gamma-ray spectrometer was used for in-situ measurements of this radioelement concentration. Studied area has covered approximately 1800 km2. In-situ measurements were performed at 1390 different locations in this field. Data for each measurement was gathered during 3 min. The map of the 40K element distribution in the studied area was produced. The high values of 40K, according to this map, were obtained in the areas of the alteration zones, between Sandýklý and Suhut in the studied area.