Because thin-layer chromatography can be affected by climatic conditions, we investigated the impact of high temperatures (33-38 degrees C) and high relative humidities (80-100%) on the applicability of Toxi-Lab thin-layer chromatographic systems for the screening of basic and neutral drugs during a 6-month climatological cycle in Jakarta, Indonesia that can be characterized as hot and humid. In general, the Rf values as observed on the plate showed substantial deviations from those in the literature obtained in moderate climates. Most substances gave higher Rf values, and the deviations (sometimes amounting to 20 Rf units) increased with increasing humidities. However, when the observed Rf values were corrected by means of a reference mixture of standard drugs that was run on each plate, much better accuracy and reproducibility were obtained. Thus, these corrected Rcf values were compatible with the existing databases developed in temperate climates. These results are in line with earlier studies carried out under hot and dry conditions. In the earlier studies, the observed Rf values tended to be lower than those present in other databases, but here too, the Rf correction procedure corrected this phenomenon.