Together with a growing number of cellular telephone users increases the interest in the effect of electromagnetic fields (EMF) emitted by them on live organisms. The surveys on subjective complaints of cellular telephone users carried out in Sweden, Norway, UK, USA, New Zealand and Australia showed that head ache is the major complain, and it is more pronounced with analogue than digital telephones. Apart from head ache, fatigue and general ill-being, muscular pains and nausea are reported. Human experimental studies reveal that EMF emitted by cellular telephones may be responsible for periodical increase in arterial blood pressure, changes in electric activity of the brain. However, no changes in secretion of cerebral pituitary hormones: adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), growth hormone, prolactin (PRL), lactogenic hormone (LH), follicle-stimulating hormone (FSH) and melatonine. The animal experimental studies indicated that exposure to EMF of the microwave frequency activates the endogenous opioid system in the brain, while the studies of the brain neurotransmitter activity have not produced univocal results, some of them showed decline, others increase in acetylcholinesterase activity. In vitro studies reveal that EMF even below maximum permissible levels may induce changes in the blood-brain permeability barrier and disorders in active transport of Na+, K+ ions and release of Ca++ ions by cellular membranes. The studies carried out thus far have not produced clear-cut results, but they indicate that EMF of the microwave frequency, including the frequency emitted by cellular telephones may be responsible for various measurable biological effects. It is essential to find out whether these effects may affect human health.